Preheat the oven to 350 degrees F. Spray a 9-inch x 9-inch baking dish with cooking spray and set aside.
Spread the oats across a baking sheet and toast for 6 minutes until light brown. Reserve 1/2 cup of the toasted oats and set them aside. Place the remaining oats into a food processor and pulse until finely ground.
In a medium bowl, whisk together ground toasted oats, flour, baking powder, baking soda, and salt. Set aside.
Using a stand mixer or a hand mixer, beat together the unsalted butter, granulated sugar, and 1/4 cup of brown sugar until light and fluffy. Add the eggs one at a time, combining well after each addition. Add the vanilla and mix to combine.
Alternate adding the flour mixture and the sour cream into the wet batter, starting and ending with the flour mixture.
Pour the batter into the prepared baking dish and spread evenly.
Combine reserved 1/2 cup toasted oats, remaining 1/2 cup brown sugar, and cinnamon in a bowl. Use a pastry cutter or fork to cut in the cold butter until well blended. Sprinkle the oat topping evenly over the batter.
Bake for 38-42 minutes or until the cake tester comes out clean. Cut into 9 square and serve warm or at room temperature.
The oatmeal can be omitted from the topping if not consumed within a few hours after baking as it can get soft.