Preheat your oven to 350°F (180°C).
Line your muffin tin with muffin liners.
In a large bowl or stand mixer, cream butter and sugars together until light and fluffy.
Peel the bananas and mix them into butter/sugar mixture.
Add the eggs, milk and vanilla extract. It will be lumpy from the bananas, which is fine.
Add flour, bicarbonate of soda, baking powder, cinnamon and mixed spice to the batter.
Mix until flour is combined. Be careful not to overmix - you want things to be *just* combined.
Coat walnuts in a little flour and stir into batter.
Using the ice cream scoop, add a level scoop of the mix into the prepared liners and bake for 18-20 min, or until a skewer comes out clean.
Remove from heat and let the muffins chill on cooling rack.