- Prep: 15 min
- Cook Time: 50 min
- Total: 1 hr 5 min
- Serving: 16
- 1-1/4 cup Dutch process cocoa powder, divided
- 3/4 cup boiling water
- 1 cup sour cream
- 1/2 teaspoon baking soda
- 2 cups sifted cake flour
- 2 sticks unsalted butter, divided
- 2 cups granulated sugar
- 3 egg whites
- 2-1/2 teaspoons vanilla extract, divided
- 2-1/2 cups confectioners' sugar
- 1/2 cup milk, more or less
- Preheat oven to 300 degrees F. and grease a 13x9-inch baking dish.
- In a small bowl, whisk together 1/2 cup cocoa and boiling water and set aside.
- In a second small bowl, stir together baking soda and sour cream and set aside.
- In a large bowl, cream 1 stick butter and granulated sugar.
- Gradually mix in the sour cream mixture and cocoa mixture alternately. Beat until fluffy.
- In a second large bowl, beat the egg whites until stiff but not dry. Fold the egg whites and 1-1/2 teaspoons of vanilla into cake mixture.
- Pour batter into prepared baking dish and bake for 50 minutes or until center springs back when touched and a knife inserted in the center comes out clean.
- Let cool on wire rack.
- To prepare frosting, cream remaining butter in a small bowl. Add 3/4 cup cocoa and confectioners' sugar alternately with milk. Beat until it reaches a spreading consistency. Add more or less milk, depending on the texture you want.
- Blend in the remaining vanilla. Frost cake and slice and serve.