1/3cuppacked dark brown sugar(light brown sugar may be substituted)
3tablespoonscold, unsalted butter, cut into small pieces
For the cake:
4ouncesunsalted butter,room temperature
1cup(8 ounces) sour cream(preferably full fat)
1 1/2teaspoonspure vanilla extract
8 1/2ouncesall-purpose flour(2 cups using the Spoon & Level Method; see notes)
Preheat the oven to 325 degrees F. Lightly grease a 9x13 inch pan with baking spray or butter.
Prepare the topping: In a medium-sized bowl, stir together the oats, granulated sugar, brown sugar, cinnamon, and salt. Press the butter into the mixture with a fork until it’s evenly incorporated. Set aside.
In a stand mixer with the paddle attachment (or using an electric mixer), cream the butter and sugar until light and fluffy. Beat in the eggs, one at a time, followed by the sour cream and vanilla extract.
In a small bowl, whisk together the flour, baking soda, baking powder, and salt. With the mixer on low, slowly add in the dry ingredients, then turn the speed up until the batter is smooth (don’t over-mix).
Spread half the batter evenly into the prepared pan (note: slightly less is better than slightly more if you’re unsure), and sprinkle with half the topping. Repeat with the remaining batter and topping.
Bake for 40 minutes, or until a toothpick comes out of the center clean. Allow to cool for at least 5 minutes. Serve warm or at room temperature.