This is a moist, all-purpose sponge cake.
Heat oven to 350 degrees. Butter and flour, or line with parchment, a 10 1/4-by-15 1/4-by-3/4- inch jelly-roll pan.
Sift together flour, cocoa, salt, and baking powder. Set aside.
Beat together egg yolks and sugar until fluffy. Beat in boiling water and vanilla.
Fold in dry ingredients in three additions.
Beat egg whites until stiff but not dry and whisk a quarter of them into batter. Fold in remainder of whites and spread batter evenly in pan. Bake about 15 minutes or until cake springs back when gently pressed. Cool in pan.