Preheat oven to 180/160 fan. Line a cupcake tin with 12 cases. Then place all the ingredients in a mixing bowl. Beat until everything is mixed together.
Divide cake mixture between cases and cook in the oven for 18-20, or until a toothpick come out clean.
Whilst cupcakes are cooking put the butter in the mixer and mix until very pale for a few minutes. Add the peanut butter and half of the icing sugar and mix again. Then add the rest of the icing sugar. If looking a little thick add a tbsp of milk.
Once cakes have cooled ice them with the buttercream and eat!