Pour a thick layer of white batter into the pan, then drop nice dollops of chocolate batter over it in various places. Repeat the process until there is no more of either batter left. The take a knife and swirl around to marbleize. The white batter should be thoroughly covering the pan every time you pour it in. It should get used up in two layers, I should thing.
Tb – did I not say this was an awesome cake?