Preheat the oven to 180C/160C Fan/Gas 4. Line a baking tray with paper cupcake cases.
For the cupcakes, beat together the butter and caster sugar using a wooden spoon until light and creamy. Stir in the eggs, one at a time, until they are completely mixed in.
Mash the bananas using a fork. Roughly chop the pecans. Stir both into the egg mixture until evenly mixed.
Fold the flour into the wet ingredients until completely combined.
Divide the mixture between the paper cupcake cases. Bake in the oven for 15 minutes, or until risen and golden. Leave to cool a little, then transfer to a wire rack to cool completely.
Meanwhile, make the icing. Beat together the butter, icing sugar and milk. Add a spoonful to the top of each cupcake and spread out using a butter knife.