Preheat your oven at 400°F (200°C) first, and prepare 1 muffins liner in a muffin pan (or grease only 1 cavity).
With a rubber spatula or a wooden spoon in a medium bowl, mix the flour, cocoa powder and baking powder. Set aside.
In another bowl, mash the ripe banana with a fork. Whisk in the egg, cashew butter, golden syrup and buttermilk.
Fold the dry ingredients in the wet ingredients, and mix everything together with your rubber spatula or wooden spoon. DO NOT OVERMIX the batter. 10 to 15 seconds are enough to combine the two preparations.
With an ice-cream scoop, fill the liners with the batter.
Bake for 5 minutes.
Lower your oven temperature to 350°F (180°C) and bake for a further 15 minutes.
Let cool before enjoying!