Preheat the oven to 350 degrees F. Grease an 8×8 pan with coconut oil.
Mix almond flour, oat flour, coconut flour, baking powder, cinnamon, pumpkin pie spice, and salt in a large bowl. Whisk to remove any flour clumps.
Add vinegar to a 1 cup measure. Fill the rest of the measure up with milk. Allow to sit for 5-10 minutes, or until it starts to curdle. This is your homemade buttermilk.
Add coconut oil, peanut butter, banana, vanilla, and maple syrup, and mix to combine. Add “buttermilk” and mix.
Pour your wet ingredients into your dry ingredients and mix well. Pour into your prepared pan and top with the coconut sugar/cinnamon mixture if desired.
Bake for 28-30 minutes, or until the sides start to brown and pull away from the pan and a toothpick inserted into the middle comes out clean. Allow to cool on a cooling rack, then invert onto a large plate to remove the cake from the pan (the bottom side will now be the top). Cut into 9 pieces when cool.
Make the glaze: Mix peanut butter, maple syrup, cinnamon, and just enough almond milk to thin the mixture to a glaze consistency. Pour over each piece of cake immediately before serving. Store unglazed cake at room temperature.