Prepare. Preheat your oven to 350°F (175°C). Grease an 8-inch square baking pan and line it with parchment paper, leaving a slight overhang for easy removal.
Dry Ingredients. In a medium bowl, use a whisk to combine together the flour, baking powder, and salt. Also, in a small bowl, combine together the sugar and cinnamon needed for the topping.
Cream. In a large mixing bowl, beat together the mashed banana, softened coconut oil, granulated sugar, and brown sugar on medium speed until the mixture is smooth and creamy (about a minute or two). Then beat in the vanilla extract, ground cinnamon, and nutmeg.
Combine Scrape down the sides of the mixing bowl and then set the mixer speed to low. Gradually add the dry ingredients from the second bowl to the large mixing bowl and mix until just combined and no dry streaks remain. The batter will appear to be thick.
Transfer. Use a rubber spatula, or similar item, to evenly spread the batter from the mixing bowl in the prepared baking pan. Sprinkle the topping mixture from the second step evenly over the batter.
Bake Bake for 25–28 minutes, or until the center is set and the edges are lightly golden brown. A toothpick inserted into the center should come out with just a few moist crumbs attached.
Cool.Let the bars cool and set in the pan for 10 minutes. Then use the parchment paper to lift the bars from the pan and transfer them to a wire cooling rack to cool completely before slicing and serving.