Beat the room temperature butter with the caster sugar and soft light brown sugar.
Beat in each egg, one at a time.
Fold in the flour and mashed bananas (do this by hand if you've been using a mixer).
Share between 12 paper muffin cases in a muffin tin.
Bake at 180C for about 19 minutes, or until very pale golden brown.
Whilst they are baking, beat the butter, with the icing sugar until well combined.
Beat in the peanut butter.
Add a teaspoon at a time of boiling water until the icing is a light, fluffy consistency.
When the cupcakes are totally cool, pipe the icing onto the cakes or swirl on with a knife.
Top with some salted peanuts.
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