Preheat the oven to 350F and line a 9x13 in dish with cooking spray and parchment paper.
Whisk milk with lemon juice and set aside.
Mix oat flour, baking powder, baking soda, salt, and cinnamon in a bowl.
Mash bananas in a separate bowl and set aside.
Cream together coconut oil and coconut sugar with an electric mixer. Mix in eggs, honey, and vanilla extract.
Fold in 1/4 of the dry ingredients, mix them until combined, and then add 1/4 of sour milk then mix again. Repeat alternating dry ingredients mix and sour milk until everything is incorporated.
Fold in the mashed banana using a spatula (not a hand mixer). Pour mixture into your dish and bake for 45 minutes or until an inserted toothpick comes back clean.
While your cake is baking, make the frosting.
Add all of your frosting ingredients to a bowl and mix with a hand mixer until evenly combined. Store in the fridge (covered) while you wait.
Let the cake cool for one hour on a cooling rack, then add your frosting, slice, and enjoy!
**Store in the fridge.