Preheat oven to 350 degrees F.
Line an 8×8-inch baking pan with parchment paper.
Place the banana and brown sugar in the large bowl of a stand mixer. (Or you can use a hand blender.) Mix medium speed until the banana and sugar are creamy and mostly smooth. It doesn’t have to be 100% smooth, a little banana texture is okay. Add the eggs and mix until smooth.
Add the coconut oil, coconut flour, oat flour, vanilla, baking powder and salt. Mix until smooth. Finally add 1/2 cup chocolate chunks and fold them in. Transfer the batter to the prepared pan and smooth out. Bake for 25 minutes, until just golden and set.
The blondies should still be soft but just cooked through. Remove from the oven and let COOL COMPLETELY. This ensures they set up. Melt remaining 2 tablespoons of chocolate in the microwave, then drizzle on top of the bars. To store, keep in refrigerator for up to 4 days.