Preheat your oven to 180 degrees Celsius.
Line a muffin tin with with 12 muffin cases.
Mash 300g of your banana in a large bowl with a fork.
Mix in the granulated sweetener and egg with a wooden spoon until smooth.
Stir in the melted butter.
Sift in the flour, baking powder, baking soda and add the salt, cinnamon and 60g of the walnuts.
Mix everything until just combined. Don’t overmix – just enough until the flour disappears.
Spoon the batter evenly in to each muffin case.
Slice the remaining 50g of banana into 12 slices and top each muffin with a slice.
Sprinkle on the leftover 10g of nuts.
Bake for about 20 minutes until the muffins are golden on top and a knife/toothpick comes out clean.
Allow the muffins to cool in their tray before storing in airtight container at room temperature for 4 days