Preheat oven to 350 F.
In a bowl cream the butter and sugar.
Add the egg and egg yolk and mix until fully combined.
Incorporate the vanilla extract.
In a separate bowl, mix the flour and baking powder.
Transfer ¼ of the flour mix to the liquid mix.
Mix well and add milk.
Repeat until all the flour and milk are used.
Slice the banana and add half of it to the mix.
Pour the batter into a greased muffin tray.
Top each muffin with a banana coin.
Bake for 30-40 minutes or until done.
Remove from the oven and let them cool down for 10 minutes.
Serve and enjoy.