Preheat oven to 325 degrees F (165 degrees C). Grease and flour three 9 inch round cake pans.
In a large bowl, cream together the butter and sugar. Beat in the egg yolks, one at a time. Stir in 2 teaspoons vanilla. Combine the flour, cocoa, baking powder and salt, stir into the creamed mixture alternately with the milk.
In a large clean glass or metal bowl, whip egg whites until soft peaks form. Fold 1/3 of whites into batter to lighten it, then quickly fold in remaining whites until no streaks remain. Divide batter into the 3 prepared pans.
Bake for 20 to 25 minutes in the preheated oven, until a toothpick inserted into the center of the cake comes out clean. Cool in pans on a wire rack.
To make the frosting: Melt the chocolate chips in the top of a double boiler, stirring occasionally until smooth. Remove from heat and stir in the sour cream, 1 teaspoon vanilla and pinch of salt. Frosting can be used warm to fill and frost the 3 layers.