Preheat oven to 350* F. Prepare a 9x9 or 8x8 pan by greasing with coconut oil or pan spray and inserting a parchment paper sling for easy removal.
In a mixing bowl, combine melted coconut oil, sugar and cocoa powder wish a whisk. Mix until everything is well combined.
Add mashed banana and vanilla into the bowl and whisk again to combine.
Switch to a spatula for stirring, and add flour, salt and baking powder. Fold together until just combined.
Reserve 2 tablespoons of the chopped chocolate for topping and add the remaining chocolate into the batter. Stir to distribute.
Scrape the batter into the prepared 9x9 or 8x8 pan and spread the batter out evenly. Top with banana slices if desired and the reserved chopped chocolate.
Bake in a 350* oven for 27-30 minutes. The center of the brownie should feel set but still soft to the touch.
Allow to cool completely before using the parchment sling to remove the brownies from the pan and cutting into 16 squares. Store at room temp in an airtight container for up to 4 days or freeze for up to 1 month.