Preheat the oven to 350 degrees F. Line an 8 x 8 baking pan with parchment paper -- otherwise your cake will likely not come out clean.
In a large bowl, stir together the bananas, egg, vanilla, and coconut oil (measured when completely melted). Stir until completely combined.
Add in the cornstarch, baking powder baking soda, cinnamon, and allspice if desired.. Stir until completely combined. (Here's where you can increase or decrease spices to personal preference -- if you are wary of too much spice add a little less and if you LOVE spice increase them.)
Stir in the brown sugar and oat flour (blend regular old fashioned oats or quick oats in a blender or food processor until they resemble flour -- blend well so you don't have any weird oat chunks! Measure AFTER blending and not before).
Stir in chocolate chips or nuts
Bake for 25-30 mins at 350 degrees or until a fork when inserted comes out clean. Be careful to not over bake as it will make this cake less tender and less flavorful. Allow to cool.
Optional Frosting;
In a microwave safe bowl combine the chocolate chips and milk. Microwave in bursts of 15 seconds, stirring for 15 seconds in between each burst until the chocolate is completely melted.
Whisk in the powdered sugar.
Start with 3/4 a cup and add as needed. Note that the frosting thickens up a lot after being refrigerated.
Refrigerate the frosting (covered in an airtight container) for at least 30 minutes.
Cover the completely cooled cake with the frosting.