Step 1

Preheat oven to 325 degrees. Butter and flour a 9-inch springform pan. Melt butter and chocolate in a bowl set over a saucepan of simmering water, stirring until smooth. Remove from heat, and whisk in 3/4 cup sugar. Whisk in egg yolk, then cocoa powder and salt.

Step 2

Beat egg whites with a mixer on medium speed until foamy. Raise speed to medium-high, gradually beat in remaining 2 tablespoons sugar, and beat until soft, glossy peaks form, about 4 minutes.

Step 3

Fold flour into chocolate mixture with a rubber spatula, then fold in egg whites. Pour batter into pan, and bake until set, about 33 minutes. Let cake cool completely in pan on a wire rack. Remove side of pan, and dust top of cake with cocoa powder.