The cake in a cup has been done countless times before (and I love it) but the idea of having two separate flavours separate, but together, in one mug was too good an idea to not do.
I chose to have a peanut butter layer at the bottom and then a coffee layer at the top. I chose these seemingly bizarre flavours both because I was feeling a bit edgy and because I am seriously running out of ingredients - our fridge is looking pretty barren with only the carcasses of out-of-date yoghurts lurking deep within.
Having said this, the cake was pretty fab. Substitute whatever you like. Here is how you do it!