Heat the oven to 190C/170C Fan/Gas 5 and put paper cases into a muffin tin.
Melt the butter and allow to cool. Mash the bananas well. Sift the flour, baking powder, bicarbonate of soda, salt, cinnamon and nutmeg together in a large bowl, add caster sugar and stir through.
Beat together the eggs, vanilla extract, melted butter and milk in a second bowl. Add the mashed banana and stir through.
Make a well in the centre of the dry ingredients and add the egg mixture, stirring roughly until it is a lumpy paste.
Then spoon the mixture into the cases until almost full. Top each one a slice of banana
Bake for 25 minutes, or until the muffins come away from the side of the pan when touched. Rest the muffin tray on a wire rack for 5 minutes, then remove the muffins and leave on the rack for another 5 minutes before serving.