Instructions for Cake
Preheat Oven to 325 degrees F.
Grease 2 9-inch cake pans or 3 6-inch cake pans (which I use)
Line with parchment paper and grease parchment paper.
Put white chocolate, butter, oil in double boiler (or use a stainless bowl over saucepan of simmering water).
Stir until melted. Set aside to cool.
Sift flour, salt, baking powder, baking soda, xanthum gum. Set aside.
Whip egg whites with cream of tarter on medium low speed until frothy. Add 1/2 cup of sugar and whip until you have stiff peaks. Transfer the egg whites to a cold clean bowl.
Clean mixer and add egg yolks and vanilla until well blended.
Add remaining sugar. Mix well.
Add chocolate and sour cream mixture to egg yolk mixture.
Turn mixer to low; add flour mixture and blend until completely combined.
Add egg whites, using a rubber spatula and fold in.
Divide cake pan evenly. I weigh my batter between pans.
Bake until cake pans pull away from pan.
If using 6 inch pans, bake for 25 to 30 minutes.
If using 9 inch, bake 30 to 32 minutes.
Instructions for Frosting
Whip butter until smooth.
Sift cocoa powder. Sift confectioner's sugar.
Add cocoa powder, salt, and sugar together. Mix well.
Add to butter a cup at a time on medium speed.
Add heavy cream, a little at a time.
You can make frosting two days in advance and refrigerate.
Or, you can make and freeze until needed.
Add sprinkles as soon as the frosting is finished.