Heat the oven to 190C/170C Fan/Gas 5. 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.
With a fork, 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 with a fork (don't over mix) until it is a lumpy paste.
Set paper cases into the moulds, or grease the moulds well with a little extra oil or butter and spoon in the mixture until almost full. Top each one with a walnut kernel if you like.
Bake for 20–25 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.
Serve the muffins with sliced banana and a dollop of creme fraiche, scattered with walnuts and drizzled with honey (or with Caramel yoghurt).