A Chocolate Cake That's Got It All

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Butter, for coating cake pans

2 cups sugar

1 3/4 cups cake flour

2 tablespoons cornstarch

1 teaspoon baking soda

3/4 teaspoon salt

2/3 cup unsweetened cocoa, preferably Dutch process

1 teaspoon instant coffee

3/4 cup boiling water

1/2 cup sour cream

1 teaspoon vanilla extract

4 large egg whites, at room temperature (you may want to save yolks to use in the frosting)

14 tablespoons (1 3/4 sticks) unsalted butter, melted, but not hot

Soft and Creamy Double Chocolate Frosting, recipe follows

Special equipment: 2 (8-inch) cake pans

Soft and Creamy Double Chocolate Frosting:

1/2 cup egg substitute, or 4 egg yolks (left over from cake), with enough milk added to equal 1/2 cup, plus additional milk, if necessary

1/2 cup unsweetened cocoa

1/2 teaspoon instant coffee

1 teaspoon vanilla extract

1/4 teaspoon salt

8 tablespoons (1 stick) unsalted butter, at room temperature

4 ounces unsweetened chocolate, melted

3 cups confectioners' sugar (sifted if it contains hard lumps)

4 teaspoons light corn syrup