Preheat oven to 170° celcius. Line a muffin tin with 6 giant muffin liners.
Place sliced butter ( butter is sliced so that it melts faster and evenly) in a saucepan over medium to low heat. Slowly melt the butter until it is foamy on the surface. Keep stirring until light brown specks begin to form and you smell a nutty aroma. Remove from heat and and pour into a small bowl with all the bits. Leave it to cool .
Puree the bananas. In a large mixing bowl, sift together flour, baking powder, baking soda, nutmeg and salt. Mix in the sugar. In a jug whisk, the eggs, pureed bananas, yoghurt, vanilla extract and browned butter. Fold the dry ingredients into the wet.
Fill each liners 3/4 full. Melt the nutella in the microwave for 20-30 seconds. Drizzle each muffin with 1 tablespoons of nutella and use a toothpick or knife to gently swirl the batter.
Bake for 20-25 minutes or until a knife inserted in the centre of the muffin comes out clean.
Remove from the oven and leave to cool for 20 minutes on a cooking rack before serving.