- 6 tablespoons salted butter
- 2/3 cup treacle or mild molasses (not blackstrap)
- 1/4 cup sugar
- Scant 1/2 cup water
- 1 cup all-purpose flour
- 1 teaspoon baking powder
- 1 teaspoon ground ginger
- 1 large egg, separated
- Confectioners sugar for dusting
- 1 cup chilled heavy cream
- 1/3 cup chopped crystallized ginger
- Preheat oven to 350°F with rack in middle. Line bottom and sides of a buttered 15- by 10-inch shallow baking pan (1 inch deep) with a sheet of parchment paper and butter parchment.
- Melt butter with treacle, sugar, and water in a small saucepan over medium heat, stirring, then cool to room temperature, stirring occasionally.
- Whisk together flour, baking powder, and ground ginger in a large bowl. Whisk in butter mixture and egg yolk until smooth.
- Beat egg white in a medium bowl with an electric mixer until it just holds stiff peaks, then fold into batter gently but thoroughly.
- Pour into baking pan, spreading evenly, and bake until top is golden and firm to the touch, 18 to 22 minutes. Cool cake in pan 5 minutes, then cover cake with a slightly damp kitchen towel and cool completely, about 1 hour.
- Dust cake generously with confectioners sugar, then cover with a sheet of parchment, then with a baking sheet or rack. Invert cake and peel off parchment now on top.
- Beat cream until it just holds stiff peaks, then spread evenly over top of cake, leaving a 1/2-inch border. Sprinkle crystallized ginger over cream. Roll up cake, starting from a short side, to make a 10-inch log, removing parchment as you go.
- If desired, slices of roulade can be frozen (up to 1 week), then lightly sprinkled with turbinado sugar (such as Sugar in the Raw) and broiled (do not thaw first) in a shallow baking pan 5 to 6 inches from heat until golden, about 3 minutes.