This is a lot like a sheet of chocolate sponge cake.
3/4 cup sugar
6 ounces semisweet chocolate, melted
2 tablespoons strong coffee
1 teaspoon vanilla extract
1/2 cup unsweetened cocoa powder
Preheat oven to 375 degrees. Grease an 11 x 16 jelly roll pan or a 9 x 13 pan and line with parchment paper.
Separate eggs. Beat egg whites with salt until they hold soft peaks. Add sugar, a tablespoon at a time, beating after each addition. Continue beating until mixture is very firm, about 5 minutes in all.
Stir egg yolks with a fork to break them up. Stir in melted chocolate, coffee, and vanilla. Fold 1/4 of the stiffly beaten egg whites into chocolate mixture. Pour back over remaining egg whites. Fold all gently together. Pour into prepared pan and spread evenly.
Bake in preheated oven for 10 minutes. Reduce heat to 350 degrees and continue baking about 5 minutes longer, or until top of pastry is firm. Cool in pan.
Sift cocoa evenly over a sheet of wax paper. Turn out pastry on cocoa covered paper. Carefully peel off paper lining.