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