Sour cream makes this coffee cake super moist and the best part is that it starts with a cake mix. It's so easy to make, and loaded with lots of cinnamon crumbly goodness. This breakfast cake is an instant crowd pleaser!
Preheat oven to 350 and grease a 9x13 inch baking dish with cooking spray.
First prepare the crumble. Whisk together flour, brown sugar, sugar, salt, and cinnamon. Cut in chilled butter until crumbly. Set aside.
Next prepare the cake. Combine cake mix, egg, sour cream, melted butter, and vanilla and mix until combined. It's okay if the batter still has some lumps.
Spread half the cake batter in the bottom of the greased baking dish. Sprinkle with half of the crumble mixture. Grease a rubber spatula and gently spread the remaining batter over the crumble (this takes a little finessing but it's worth it if you just work with it - see note). Top with remaining crumble.
Bake for 30-35 minutes until an inserted toothpick comes out clean. Serve warm.
If you don't want to worry about working with the batter (which is thick) to make the layers, you can spread all of the batter in the pan, then top with all of the crumble and bake as directed.