Dissolve cocoa in boiling water, cool. In a large bowl beat butter and sugar until light and fluffy, add eggs one at a time beating after each egg is added,add vanilla.
Combine the flour, baking soda, baking powder and salt in a bowl.
Gradually add flour mixture alternating with sour cream, add the cocoa mixture and mix well.
Pour into 2 10 inch greased and floured cake pans, bake at 350 for 35 minutes or until a toothpick comes out clean.
NOTE This cake also can be baked in 3 standard 9 inch pans for 30 minutes, I changed the pan size for this 2 layer cake as its my best chocolate cake recipe!
Cool cake in pans 10 minites on rack then run a knife to looten edges and turn out and cool.
FOR PASTRY CREAM FILLING
In a large bowl combine sugar, flour, cornstarch and egg yolks, beat on high speed 2 minutes until thickened.
Meanwhile in a saucepan heat milk to a simmer, gradually pour one third of the hot milk into the egg mixture, whisking all the time,pour mixture back into saucepan still whisking,scraping sides to keep smooth.Cook over low ro medium heat until it begins to thicken and bubble cook 60 seconds more. Pour into a bowl press parchment paper on surface of pastry cream to prevent a skin and chill until cold. This can be done a day or two in advance.
FOR WHITE CHOCOLATE GLAZE
Hrat cream to a simmer, remove from heat add white chocolte, let sit a minute and than stir until smooth
TO ASSEMBLE CAKE
Spread cold pastry cream between cooled cake layers. Drizzle lukewarm glaze over letting it drizzle down the sides.
Garnish with chocolate shavings and white chocolate sprinkles and chill.