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.
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.
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.
Cake can be stored in pan at room temperature, wrapped in plastic wrap (do not let plastic touch cake), up to 1 day.