Makes 1/2 sheet pan
6oz. semi-sweet chocolate
2Tbl. unsalted butter (cold)
2Tbl. cold water
1/4C all-purpose flour
1/4C cocoa powder
pinch of salt
6 eggs, seperated
1tsp. vanilla extract
1/8tsp. cream of tartar
Line pan with parchment paper, and spray with non-stick spray; dust with flour and tap out the excess. Preheat oven to 400F (or 375F convection bake).
Over a double boiler, gently melt the chocolate, butter and water together just until melted – stir gently to combine (be careful to not allow the water inside the bowl to boil). Sift the flour, cocoa and salt together.
With an electric mixer, whisk together the egg yolks and half the sugar for 5 minutes on high, until the mixture is at least tripled in volume, pale yellow and thick – add the vanilla extract. Pour the mixture into a large mixing bowl and wash out the electric mixer bowl and whisk very well. Stir the melted chocolate mixture into the egg yolk mixture.
In the clean electric mixer bowl, whisk the egg whites and cream of tartar until foamy. Add about 1tsp of the remaining sugar and continue to beat on medium-high until soft peaks form, then slowly drizzle in the rest of the sugar and continue to beat until the whites are glossy and smooth, but hold stiff peaks. Do not over beat (if the whites become dry, grainy and chunky, they are over beaten).
With a rubber spatula, gently fold about ¼ of the meringue mixture into the chocolate/yolk mixture to lighten it. Fold in the remaining meringue mixture until almost no streaks of white remain. Sift the dry ingredients over the top and quickly, but gently fold in. Pour the batter into the prepared pan and quickly even the top with an offset spatula.
Bake until the center of the cake springs back to the touch, about 8-10min. Cool the cake in the pan on a wire rack for 5 minutes before flipping out onto a piece of parchment paper that has been generously sprayed with non-stick spray, or a clean kitchen towel. Peel off the parchment paper the cake was baked on and cool before filling/rolling.