Preheat your oven to 180c and line a brownie tin with baking paper.
Mash the banana until smooth, then beat in the sugar, 1/4 cup of the butter and nut butter. Whisk in the eggs, then add your dry ingredients.
Melt the chocolate and remaining butter together, then pour this into your batter whilst mixing to create a smooth, velvety texture. Fold through your chocolate chunks and transfer into the baking tin.
Pop these into the oven for around 11 minutes, then reduce the heat to 170c and bake for a further 7 minutes.
Once your brownies are cooled, you can either top these with marshmallows or make your own frosting. To make the frosting, in a Bain Marie gently heat the sugar, egg whites, cream of tater and salt whilst whisking until all of the sugar is dissolved. Remove from the heat and whisk to firm, glossy peaks with an electric whisk (should take around 8 minutes).
Spread this over your brownies and use a kitchen torch to toast/or pop under the grill. Alternatively, layer on your marshmallows and light toast these instead! Slice into squares and enjoy!