Make the streusel: Add Streusel Topping ingredients to a small bowl. Using a fork, stir and fold until well incorporated. Set aside.
Make the banana coffee cake: Peel bananas and add to a large mixing bowl. Using a pastry cutter or fork, mash bananas until smooth. Add the coconut oil, maple syrup, flax egg and vanilla. Whisk until well incorporated.
Add oat flour, almond meal, baking powder, baking soda, cinnamon (if using) and salt. Whisk together until just incorporated, making sure no flour patches remain. Batter will be a bit thick.
Pour batter evenly into prepared pan. Using a rubber spatula, spread into an even layer. Sprinkle streusel evenly over batter.
Bake for 30-40 minutes. Mine took 35 minutes. Test for doneness by sticking a toothpick into the center of the cake. The toothpick will come out clean when the cake is done.
Storage notes: Cool completely, then store in an airtight container for up to 1 week at room temperature in a cool, dark environment. If stored in the refrigerator, topping will harden. Allow to soften at room temperature before enjoying, if desired.This coffee cake freezes well. Slice first, then store in an airtight container in the freezer for up to 1 month. Allow to thaw at room temperature for 30-40 minutes.Nutrition Information: is an estimate based on the best-selling, most common brands per ingredient. Also, it's estimated based on 16 servings per recipe.