Heat milk in small pan until bubbles begin to appear around edges. Remove from heat.
Mix cocoa and white sugar together in a small bowl and slowly beat in warm milk. Let cool.
Cream the butter with the brown sugar. Beat in the eggs, sour cream, and vanilla. Add cocoa mixture.
Mix dry ingredients together and gently blend into butter mixture.
Do not overheat.
Turn into 2 well-greased and floured 9-inch layer cake pans, and bake 25 to 30 minutes, until cake shrinks slightly from sides of pans and springs back when touched gently in the center. Cool on a rack for a few minutes, then turn out onto rack.
Wait until completely cool before frosting.
Combine egg whites, sugar, water, cream of tartar, and salt in top of double boiler. Set over simmering water and beat with an electric mixer for about 5 minutes, until soft peaks are formed. Remove from heat and stir in vanilla. Keep beating until frosting is stiff enough to spread. Use immediately. This looks like a lot, but use it all; it is enough to fill and frost the cake.