Beat the butter with the brown sugar until smooth and creamy. Add the egg and continue to beat until smooth. Then beat through the mashed up banana; don't worry if there are some banana chunks in there.
Combine the wholemeal flour, ground almonds & baking powder. Fold through the banana mixture until smooth. Then fold through the milk until all smooth.
Spoon mixture into a greased cake tin and bake at 170C (340F) for around 40-45 minutes. Check it at the 30 minute mark cause you don't want to over cook this. The top should be nice and golden.
For the frosting, simply whisk together all ingredients until nice and smooth. Add a bit more ground almonds to thicken it up if you like. Spread over cooled cake.