Cinnamon Swirl

1. Preheat the oven to 325F/163C and line a 9-inch square baking pan with parchment so that the parchment paper overhangs on the sides, this will help you pull the baked cake out of the pan later. Grease the parchment with non-stick baking spray and set aside.
2. In a small bowl whisk together the brown sugar and cinnamon until fully combined, set aside.

Streusel Topping

1. In a medium bowl whisk together the flour, brown sugar, and cinnamon. Pour in melted butter and use a spatula to mix it all together to moisten everything, this will take a couple of minutes, you want a wet sand texture, thick clumps are good. Set aside in the refrigerator while you prepare the cake.

Cake

1. In a large bowl whisk together the flour, baking soda, baking powder, and salt. Set aside.
2. In the bowl of an electric stand mixer fitted with a paddle attachment or a large bowl with a handheld electric whisk beat the sugars and butter together for 2-3 minutes, scrape down the bowl after the first minute then continue mixing. You want a light fluffy mixture. Mix in the oil and vanilla until fully combined.
3. Add the eggs, scraping down the bowl and making sure the eggs are fully combined, there should be no trace of egg yolk. Mix in the mashed bananas, the mixture may look curdled, but that is okay. 
4. Gently fold in 1/3 of the dry ingredients until just almost combined, then gently fold in 1/2 of the sour cream until just combined, then repeat with 1/3 of the dry, the rest of the sour cream, and finish with the remaining flour. Do not over-mix.
5. Pour 1/2 of the batter into the prepared pan then sprinkle all of the cinnamon swirl mixture on top in an even layer. Scoop spoonfuls of the batter on top of the cinnamon swirl layer and use an angled spatula to spread the cake batter into an even layer, if some of the cinnamon swirls on top it is okay. Sprinkle all of the crumble on top into an even layer. Bake for 40-50 minutes, since the cake is covered in a crumble it is best to check by using a toothpick test, insert a toothpick into the center of the cake, moist crumbs are okay but if there is any wet batter the cake will need to cook longer.
6. Let the cake cool on a wire rack for at least 30 minutes before using the parchment overhang to gently and quickly remove it from the pan. Best served the day it is made but leftovers will last in an airtight container for up to 3 days.