3tablespoonsroom temperature whole milk plus more if needed
2teaspoonsvanilla
Pinchor two of salt if using unsalted butter
Instructions
Preheat oven to 350 degrees F. Spray two 9 inch round cake pans with sides that are at least 2 inches high, with flour-added baking spray.
Combine cake mix, sugar, salt, eggs, vanilla, oil, butter, milk and water in a large bowl DO NOT ADD THE FLOUR YET. Beat with an electric mixer until smooth, scraping sides of bowl well. After you've beaten the batter, THEN add the flour and stir until mixed. The point is not to overbeat the flour :).
In a small mixing bowl, mix together cocoa powder and sour cream. Stir in 1 cup of the vanilla batter. This is tricky because the chocolate mixture gets lumpy. You may want to start with a couple of tablespoons of vanilla batter and stir until smooth, then gradually add about a cup.
Divide vanilla batter evenly among pans. Drop globs of chocolate batter over the vanilla batter. Swirl through. For deeper looking marbles, drag back of a spoon through vanilla batter and let the chocolate batter sink down a bit.
Bake for 30 minutes on center rack or until cakes test done. Let cool in pans for 10 minutes, then carefully invert. Let cool completely.
To make the frosting, melt both chocolates in microwave or double boiler and set aside to cool for about 30 minutes. Beat the butter until creamy. Stir in the cocoa power, about a cup of the powdered sugar, and the slightly cooled (it can be a little warm) melted chocolate. Scrape sides of bowl and beat well. Add remaining powdered sugar, then add milk, vanilla and salt, beating and scraping until you get the consistency you like.
Notes
For the frosting, make sure all ingredients are at room temperature and not cool at all. If ingredients are too cool, some of the chocolate may re-solidify giving you little chunks of chocolate in the frosting. It's happened to me a couple of times. People actually like the little chunks, but I don't think the frosting looks as nice when that happens.