Preheat the oven to 350 degrees F. Line a square 9x9 baking pan with parchment paper and set aside.
In a medium bowl, whisk together your dry ingredients.
In a separate bowl, combine your eggs, milk, banana, and oil. Beat these into the dry ingredients until a very thick batter begins to form.
Transfer half of the mixture into your baking pan. Whisk together the filling ingredients then sprinkle on top of the batter. Top with the rest of the batter and swirl together with a butter knife or toothpick.
Prepare the crumb topping by combining all the ingredients into a small bowl and beat until you have a sandy texture. Sprinkle the mixture evenly over the top of the cake.
Bake in the center of your warmed oven for 35 - 38 minutes. Test with a cake tester by placing it in the center of the cake. If there is nothing on the stick when you remove it, the cake is ready!