Preheat the oven to 190 degrees Celsius/375 F and line a 12 hole muffin tray with paper cups.
Brown the butter by adding 115g of unsalted butter to a small frypan on medium-high heat. Stir continuously for about 6 minutes or until after it foams and turns into a brown colour with specks of caramelised milk solids at the bottom. Leave it aside to cool.
In a large bowl, whisk together the mashed bananas, sugars, yoghurt, lemon juice and vanilla.
In a separate bowl, whisk together the flour (see notes about gluten free flour), baking powder, bicarbonate soda and salt.
Gradually add the wet ingredients into the dry ingredients using a rubber spatula to mix together.
In a separate small bowl, mix together the cinnamon sugar by whisking the extra caster sugar and ground cinnamon together and set aside to put on top of each muffin before baking in the oven.
Fill each muffin paper cup with the batter to the half way mark and sprinkle the cinnamon sugar over the top (about quarter of a teaspoon for each muffin).
Pop into the oven and bake for 22 minutes or until the muffins have baked right through.
Serve the muffins cooled with a warm cup of tea or hot chocolate.