- 1 tablespoon rice flour
- 1/2 cup brown rice flour
- 1/2 cup rice flour
- 2 teaspoons tapioca flour
- 1/2 teaspoon salt
- 4 eggs, at room temperature, separated
- 1/2 teaspoon cream of tartar
- 1 cup white sugar, divided
- 1 tablespoon lemon zest
- 1 cup grapeseed oil
- 1 cup chopped toasted walnuts
- 1 tablespoon vanilla extract
- 1 tablespoon unsweetened cocoa powder
- 1 teaspoon instant coffee powder
- Preheat oven to 350 degrees F (175 degrees C). Grease a 10-inch loaf pan; dust with 1 tablespoon rice flour.
- Mix brown rice flour, 1/2 cup rice flour, tapioca flour, and salt together in a bowl.
- Combine egg whites and cream of tartar in the bowl of a stand mixer fitted with the paddle attachment; beat for 3 to 4 minutes. Add 1/2 cup sugar gradually, continuing to beat until stiff and glossy peaks form. Transfer beaten egg whites carefully into a clean, dry bowl.
- Combine egg yolks, remaining 1/2 cup sugar, and lemon zest in the bowl of the stand mixer. Beat for 3 to 4 minutes. Add oil very slowly, continuing to beat until fully incorporated.
- Fold rice flour mixture and beaten egg whites very gently into the egg yolk mixture. Pour 3/4 of the batter into the prepared loaf pan.
- Fold walnuts, vanilla extract, cocoa powder, and coffee powder into the remaining batter. Pour into the loaf pan.
- Bake in the preheated oven until a toothpick inserted into the center comes out clean, 35 to 40 minutes. Turn off oven and let cake cool inside, about 20 minutes. Transfer cake to a wire rack and let cool completely, about 20 minutes.