Preheat your oven to 180c and line a brownie tin with baking paper.
In a mixing bowl, mash the bananas thouroughly with the back of a fork, then beat in the coconut oil and sugar.
Add the eggs one at a time and mix these into the batter.
Combine all of the dry ingredients, then fold these into the wet mixture to create a smooth batter and carefully stir in the pistachios.
Pour your brownie mix into the lined tin and bake these for around 20 minutes. Remove them from the oven and allow to cool completely on a wire wrack.
Meanwhile, for the frosting, mix together the cashew butter, honey and then add a little milk whilst mixing to create a smooth, spreadable consistency. Melt the chocolate in the microwave, then quickly whisk this into your cashew frosting.
Spread the chocolety icing over your brownies, garnish with some extra chopped pistachios and slice into squares!