To prepare chocolate sponge cake, preheat the oven to 350 degree Fahrenheit. Heat 1/4 cup milk and 2 tablespoons of butter in a small saucepan. In a large bowl, use the wire whip of an electric mixer to combine 8 eggs with 2 cups of sugar, beating on medium-high speed until the mixture turns a pale yellow color, triples in volume and turns thick, which takes around eight minutes. With the mixer still running, slowly add the milk and butter mixture.
In a mixing bowl, sift together 3/4 cup of flour, 1/2 cup cocoa powder and 1 teaspoon each salt and baking powder. Fold the flour mixture into the egg mixture, mixing until smooth, and adding 1 teaspoon of vanilla.
Grease a 1/2-sheet cake pan with 2 teaspoons of butter. Sprinkle the butter with 2 tablespoons of sugar, and pour the batter into the pan. Bake the cake until it springs back from the pan when touched, which takes around 25 minutes. Cool the cake for two minutes, and then loosen its edges with a thin knife before flipping it onto a wire rack to cool completely.Learn more about Desserts