Preheat your oven to 325 degrees F. Spray two 8-inch by 4-inch bread pans with non-stick spray, line with parchment, and spray again. Set aside.
In a medium sized bowl combine the flour, oats, cinnamon, baking powder, baking soda, and salt. Whisk to combine. Set aside.
In a stand mixer fitted with the paddle attachment, beat the butter and sugars on medium-high speed until pale in color and fluffy in texture, about 3 minutes. Add the Biscoff spread and mix an addition 1 minute on medium-high.
Lower the speed to medium-low and add the eggs, one at a time, mixing until each is fully incorporated before adding the next (about 30 seconds). Turn the mixer to medium-high and beat the mixture for an additional 2 minutes, until lighter in color. Add the vanilla and mashed bananas and stir to combine.
With the mixer on low, add the dry ingredients alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just incorporated.
Pour 17 ounces of batter in each pan. Drizzle 1/4 cup to 1/2 cup Biscoff spread over the batter. (I heated my Biscoff spread for about 30 seconds in the microwave). Top each pan with remaining batter (about 16-17 more ounces in each pan). Spread the batter with the back of a spoon to make it level.
Bake both pans together in the oven for 50 to 60 minutes, or until a toothpick inserted in the center of the cake comes out with just a few moist crumbs on it.
Let pans cool on wire rack for 20 minutes, then invert cakes onto racks, gently, peeling away parchment rounds. Let cool completely.