Marble Velvet Cake

So many people have requested a marble cake that I promised them my next cake book would include one. This is a marriage of my favorite yellow sour cream—based layer cake to a variation of my favorite chocolate sour cream—based layer cake. All you have to do is melt your favorite bittersweet chocolate, add it to part of the batter, and swirl away. To dress it up for special occasions, add the ganache glaze.