Thanks so much guys! Well, I've tried the cocoa idea (taking out a cup and adding cocoa to it then swirling in with a knife). It's in the oven - I'll let you know how it turns out. I would've used the 1 cake mix of vanilla and 1 cake mix of chocolate idea, but I'm only making an 8" right now and don't want to waste two boxes. I wonder why I can't find the Betty Crocker Supermoist Marble anywhere. Does anyone know if they've discontinued it?