Preheat oven to 300 degrees F (150 degrees C). Grease a 9x13 inch pan.
In a small bowl, mix together 1/2 cup cocoa and 3/4 cup boiling water; set aside.
In another small bowl, dissolve baking soda in the sour cream by stirring them together.
In a large bowl, cream the 1/2 cup butter and 2 cups sugar. To the butter and sugar mixture, alternately add the sifted flour and the cocoa mixture with the sour cream mixture. Beat until fluffy. Beat the egg whites until stiff and fold in the egg whites and 1 1/2 teaspoons of vanilla.
Bake in the preheated oven until a toothpick inserted near the center comes out clean, about 50 minutes. Frost with La Famiglia Chocolate Frosting (below).
To Make La Famiglia Chocolate Frosting: Cream 6 tablespoons butter or margarine in a small bowl. Add 3/4 cup cocoa and confectioner's sugar alternately with milk; beat until spreading consistency. More or less milk can be used depending on the texture you want. Blend in 1 teaspoon vanilla. This yields about 2 cups of frosting.