Chocolate stout cake with peanut butter frosting

I've been trying to keep up with the inventory idea for the last year and it's worked out fine so far: the ingredients no longer end up in the garbage and that is great. From time to time I check the contents of my pantry/freezer/fridge to make sure nothing goes to waste and a couple of days ago I found a can of Guinness beer in the cupboard; while searching for something delicious to make with