Layered Chocolate Mousse Cake Recipe

1 ½ cups (6 ounces/170 grams) cake flour
1/3 cup (1 ounce/28 grams) unsweetened natural cocoa powder (not Dutch process)
2 teaspoons baking powder
¼ teaspoon baking soda
¼ teaspoon table salt
1 cup (8 ounces/227 grams) granulated sugar
1 cup (8 fl ounces/223 ml) water
1 large egg
1/4 cup (2 fl ounces/58 ml) canola or corn oil
2 teaspoons pure vanilla extract
2 ¼ cups (18 fl ounces/525 ml) heavy cream
1/3 cup (1 ounce/28 grams) unsweetened Dutch-process cocoa powder, sifted if lumpy
14 ounces (397 grams) bittersweet chocolate, chopped
10 tablespoons (5 ounces/142 grams) unsalted butter, cut into 8 pieces
2 teaspoons pure vanilla extract, or 1 to 2 tablespoons brandy, Cointreau, or bourbon
1/4 teaspoon table salt
8 whites from large eggs (1 cup/8 fl ounces/233 ml)
2/3 cup (5 ounces/142 grams) granulated sugar
2 cups (9 ounces/255 grams) medium-fine chopped walnuts, toasted, for garnish (optional)
10- to 12-ounce (284- to 340-gram) thick block bittersweet, semisweet, milk, or white chocolate for garnish (optional)

To make the cake:
Position an oven rack on the middle rung. Heat the oven to 325 degrees (165°C). Lightly grease the bottom and sides of a 9-by-2 inch (22.75-by-5cm) round cake pan. Line the bottom with parchment and flour the sides, tapping out the excess flour.
Sift together the cake flour, cocoa, baking powder, baking soda, and salt into a large bowl. Add the sugar and whisk until well blended, measure the water in a 2-cup Pyrex measure. Add the egg, oil, and vanilla. Using a fork, mix until blended. Scrape the wet ingredients into the dry ingredients. Whisk until the dry ingredients are just moist. Resist the urge to overbeat, or the baked cake will be dense and form a dome when baking. Pour the batter into the prepared pan.
Bake until a toothpick or cake tester inserted in the center comes out clean, about 33 minutes. Transfer the pan to a rack and let cool for 20 minutes. Lightly grease a wire rack and invert the cake onto it. Lift off the pan and peel off the parchment. Let cool completely.
To make the mousse:
Pour the cream into a large saucepan and set over medium heat. Add the cocoa powder. Bring the cream to a full boil, whisking occasionally to blend in the cocoa. Slide the pan from the heat and immediately add the chocolate and butter. Whisk slowly until melted and smooth. Scrape the mixture into a large mixing bowl and set over another large mixing bowl filled with ice and some water. Add the vanilla or liquor and salt. Stir frequently with a rubber spatula until the mixture has cooled to room temperature.
While the chocolate mixture is chilling, gather together the supplies for assembling the cake. You’ll need a large, flat, serving plate, toothpicks (for cake slicing; optional), a long serrated knife, a rubber spatula, a long, metal spatula (offset is fine), and 2-cup measure (I use the kind for dry-weight measuring).
In a large bowl, beat the egg whites with an electric mixer (stand mixer fitted with the whisk attachment or handheld mixer) on medium-low speed until they are frothy. Increase the speed to medium-high and beat until the whites form soft peaks. Continue beating while gradually sprinkling in the sugar. When all the sugar has been added, increase the speed to high and whip until form, glossy peaks form.
Scoop about one-third of the whites into the cooled chocolate mixture and whisk gently until blended. Scrape the remaining whites into the chocolate and fold together gently but thoroughly with a rubber spatula. Scoop out about 2 ½ cups of the mousse, cover, and refrigerate for the final touchups and glaze, if using. (Otherwise, it’s a treat for the chef.) Cover and set aside the remaining mousse at room temperature.
To assemble the cake:
Remove the bottom of a 9-inch (22.75cm) springform pan and set the ring on a large, flat serving plate. If using toothpicks to help slice the cake, stand a ruler against the side of the cake and evenly insert 2 picks vertically along the outside edge, dividing the cake into 3 equal horizontal layers. Repeat around cake. (Think of the cake as a clock, and stick the picks in at 12, 3, 6, and 9 o’clock.) Set your hand atop the cake, position a long serrated knife, just barely above the top toothpick, and, using a sawing motion with long, gentle strokes, slice off the top layer of the cake. Flip the top layer upside down and place it in the bottom of the springform ring. Using the toothpicks as a guide, repeat to cut the remaining cake into 2 layers. Set aside. Scoop about 2 cups of the mousse onto the first cake layer in the ring and gently spread to cover. Place the next (middle) cake layer onto the mousse and press gently to level. This will push the mousse to the edge of the pan. Spread about 2 more cups of the mousse over the second layer. Turn the remaining (bottom) cake layer upside down and place it on top of the mousse. Press gently on the layer to level. Spread the remaining mousse on the final cake layer and smooth the top. The cake should fill the springform ring. Cover the assembled cake with plastic wrap and refrigerate until well chilled and the mousse is firm, about 6 hours.
To garnish the cake:
Remove the chilled cake from the fridge and uncover. Warm a long, thin knife or metal spatula and slide it between the cake and the ring to loosen the cake. Carefully unlock the springform clasp and slowly release it until it is open all the way. Lift the ring away from the cake. Clean the plate edge, if necessary, and slide thin strips of parchment or foil slightly under the cake if you’d like to keep the plate clean while you garnish the cake. Check the sides of the cake. If they aren’t completely covered with mousse, use a small metal spatula and about 1 cup of the reserved mousse for cover any bare patches and smooth the sides.
To garnish with chocolate glaze:
Melt about 1 cup of the remaining reserved mousse in a double boiler or in a 2-cup Pyrex measure in the microwave. Pour the glaze onto the top center of the chilled cake until a large pool forms and begins to spread to the edges. The top should be completely covered and the glaze should drip down the sides of the cake in a few spots and form small pools on the plate. If there are any bald spots around the edges, drizzle a little glaze on them to cover. This will also coax the glaze over the edge. Serve immediately, or cover and refrigerate until serving.
To garnish with walnuts:
Scoop up some of the walnuts with one hand and gently press them against the side of the cake. Rotate the cake plate slightly and repeat. Continue turning the plate and pressing the nuts onto the cake until the sides are completely covered. Bush any extra walnuts off the plate before removing the parchment or foil strips and save for another recipe. Serve immediately, or cover and refrigerate until serving.
To garnish with chocolate shavings and curls:
Place 2 sheets of parchment or waxed paper, each 11 by 17 inches (28 by 43cm), on a work surface, then make the chocolate shavings and curls. Use a thick 10- to 12-ounce block of bittersweet, semisweet, milk, or white chocolate. The chocolate will need to be warmed slightly. Rub it with your palm or use a microwave for short bursts of about 5 seconds (one or two should do it). Applying pressure, drag a vegetable peeler across the edge of the block, letting the shavings fall onto the paper. To make curls, use the same technique as for shavings, but the chocolate needs to be a bit warmer. Rub it with your palm or use a microwave for short bursts of about 5 seconds (two should do it) until it feels just slightly warm; white chocolate will need less time. Press harder on the vegetable peeler. If the chocolate still won’t produce big curls, it isn’t warm enough, so heat it again for 5 seconds. If the chocolate melts against the peeler, let it cool a bit, and then try again. For wider curls, shave the side.
Let the shavings fall onto one sheet of paper and the curls onto another. You’ll need 1 ½ to 2 cups shavings; make enough curls to cover the sheet in an eve, single layer. Use a tablespoon to scoop up some of the shavings. Starting at the bottom of the cake and using light pressure, gently drag the spoon up the side so that the shavings stick; continue until the sides of the cake are completely covered. Arrange the curls on the top. Brush any extra shavings off the plate before removing the parchment or foil strips and save for another recipe. Serve immediately, or cover and refrigerate until serving.