Prepare the cinnamon filling first by combining the brown sugar, cinnamon, and melted brown butter, then set aside.
In a bowl, add the mashed bananas, browned butter, and coconut sugar. Whisk together until mixed.
Add the egg, vanilla, and milk, and mix evenly.
Sift in the all-purpose flour and baking powder, then fold everything together with a spatula.
Pour the batter into a baking pan.
Add dollops of the cinnamon filling on top and swirl it together with a knife.
Bake for 40 minutes.
Cool the cake and add the icing made with powdered sugar and milk.
