Chocolate Sponge Cake

Preparation

1. For cake:

   Step 1

   Preheat oven to 400°F. Line the bottom of a 13x9x2" metal baking pan with parchment paper. Butter paper and sides of pan; dust with cocoa powder, tapping out excess. Sift flour and remaining 1/4 cup cocoa powder through a fine-mesh sieve into a small bowl. Repeat sifting 2 more times. Set flour mixture aside. Crack 2 eggs into a large deep bowl. Separate remaining 2 eggs, adding yolks to bowl with whole eggs and placing whites in a medium deep bowl. Set whites aside.

   Step 2

   Using an electric mixer, beat whole eggs and yolks on low speed for 1 minute. Increase speed to medium; gradually add 1/2 cup sugar. Beat until mixture is thick and pale, about 3 minutes. Beat in vanilla.

   Step 3

   Using clean, dry beaters, beat egg whites until foamy. Sprinkle 1 teaspoon sugar over. Continue to beat until soft peaks form, about 1 minute. Sprinkle remaining 5 teaspoons sugar over and beat until meringue is thick and glossy, about 30 seconds.

   Step 4

   Using a rubber spatula, fold half of meringue into egg mixture. Sift half of dry ingredients over batter and fold until just blended. Repeat with remaining meringue and dry ingredients.

   Step 5

   Scrape batter into prepared pan, spreading evenly to edges. Bake until cake springs back when pressed gently with your fingertips, 10-12 minutes.

   Step 6

   Let cake cool in pan on a wire rack. Run a knife around sides of pan to release cake.

   Step 7

   Place a cutting board on top of pan. Invert cake onto board; remove pan. Carefully peel away parchment. Using a serrated knife, trim edges to create an even layer.

2. For chocolate icing:

   Step 8

   Bring cream to a simmer in a small saucepan over medium-low heat. Remove from heat; add chocolate chips and stir until icing is smooth and glossy. Let cool to thicken slightly, about 15 minutes.

   Step 9

   Using an offset spatula, spread half of icing down the middle of the cake; smooth evenly to the edges. Cut cake crosswise into 3 pieces. Stack pieces to make 3 layers. Spread remaining icing on sides of cake. DO AHEAD: Can be made 2 days ahead. Store airtight at room temperature.