Preheat your oven to 350° and spray a large cast iron pan, or 13X9 inch pan, with nonstick spray or line with parchment paper.

Place flour, brown sugar, cinnamon, kosher salt, and cold cubed butter into the bowl of your food processor and pulse until combined into a lumpy cornmeal texture. This can also be done using your fingers, making a finger snapping motion to cut the butter into the drying ingredients.

Pour the mixture into a bowl and add the oats. Pinch together with your fingers to mix the oats in. Set aside.

Place two bananas into a large bowl and use a fork to mash. Add in sour cream, granulated sugar, eggs, and vanilla extract and whisk until combined, until there are just some small banana lumps visible.

Drizzle in melted and cooled butter and whisk to combine. Measure in your baking powder, baking soda and kosher salt and whisk one more time.

Sprinkle all purpose flour over the surface of your wet ingredients and use a spatula to fold in until the last of the flour disappears into the batter.

Transfer half of the batter into your prepared pan and smooth the top. Sprinkle half of your streusel mixture on top of the batter.

Using a spoon, or a medium ice cream scoop, dollop the remaining batter evenly over the top of the streusel layer. Use an offset spatula or silicon spatula to evenly spread the batter out. Sprinkle the rest of the streusel on top.

Bake for 45 to 50 minutes or until a skewer inserted into the middle of the cake comes back clean. Allowed to cool for at least 15 minutes before dusting with powdered sugar and serving.