Pre heat oven to 350˚ Fahrenheit. Grease a 10 inch cast iron skillet with butter or vegetable oil.
Using a stand mixer and the paddle attachment, beat the butter, granulated and brown sugar until light and fluffy. Beat in the sour cream, eggs one at a time and vanilla.
In a large bowl, whisk together the flour, baking powder, baking soda and the salt. Add the flour mixture to the butter mixture in 3 additions. Combine well but don't over-mix.
Spread 1/2 of the batter into the prepared pan, smooth with a knife or offset spatula. Sprinkle 1/2 of the crumble onto the batter. Spread and smooth the rest of the batter over the crumble. Top with the rest of the crumble, pressing in very gently.
Bake for 40-45 minutes or until a wooden skewer comes out virtually crumb free.
Whisk together the flour, brown sugar and cinnamon in a bowl. Add in the butter and mix with clean hands or a pastry cutter. The crumble should stick together and the pieces of butter should be no larger than peas.
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