Lightly butter a 9 x 13 inch cake pan / baking dish and preheat the oven to 175C / 350F (170C / 340F fan oven).
In a food mixer or food processor (or in a bowl with a hand-held electric whisk), soften the butter (about a minute), then add the sugars and cream together until light and fluffy (about 2 minutes).
With the motor / whisk running, add the eggs one by one until combined, then whisk in the mashed banana, vanilla, and milk.
In a separate bowl, sift the flour together with the baking powder, baking soda, salt, and cinnamon.
Slowly add the flour mixture to the wet ingredients (pour wet mixture into a bowl if you’ve used a food processor), mixing until just combined.
Pour the batter into the cake pan / baking dish and bake for 30 minutes or until a skewer inserted into the centre of the cake comes out pretty clean. Leave to cool in the pan.
Meanwhile, make the frosting. Combine the butter, milk, peanut butter and vanilla in a saucepan and heat and stir until melted together.
Slowly add the powdered / icing sugar until you have a smooth and thick frosting. Pour over the cake and spread evenly.
Sprinkle over the chopped peanut butter cups (if using). Pop the whole pan in the fridge or freezer for a short while to set the frosting a bit, then slice and serve.
Notes
Don't have any buttermilk?
If you don’t have any buttermilk and don’t want to buy any just for this recipe, simply add about ¾ tablespoon vinegar or lemon juice to ordinary milk and let stand for 5 minutes (I always do it this way, as buttermilk isn’t a standard ingredient here in the UK).
Mixing the batter
Don't overmix after you add the dry ingredients to the wet. Just mix the flour in until just combined. A bit lumpy is fine!
Cooking time
Begin checking the cake after 25 minutes as cooking time will vary a little depending on your oven.
Decorating the cake
Feel free to use Reese’s Pieces or Peanut Butter M&Ms instead of Peanut Butter Cups for decoration - or just skip this final step altogether!