I wasn't intending to post about this, as I made it in a hurry and didn't think the idea had worked very well. However, when I cut into the cake, it looked so pretty that I thought it was worth writing about it.
Chocolate Mayonnaise Cake I made a while ago, but I made a plain vanilla flavoured cake by leaving out the cocoa and increasing the flour. Then I stirred a cocoa paste made from 30g cocoa and some boiling water into one portion of the batter (a bit less than half, by weight).
At this point I realised the two batters were too thin to marble properly, so I poured the vanilla batter into the cake tin, added the chocolate batter on top, and attempted to swirl the two together with a fork. I had no idea how much (or little) the two batters had mixed until the cake was cut, but I was pleasantly surprised by the result!
This didn't turn out to be a spectacularly good cake - it's more the sort of thing you have on hand for people who expect a daily portion of cake, rather than something you bake for guests - but it is a moist, tasty sponge cake, with an acceptable chocolate flavour. I don't think anyone would guess it had no butter in it!