It's no secret that the best part of a crumb cake is, well, the crumb! Unlike other versions, this sheet pan dessert is half cake and half crumb topping, for more deliciousness in every slice. The cake part is perfectly sweet, with just the right amount of tang from the sour cream, while the topping is mega buttery and spiked with cinnamon. Win-win!
For the crumb: Combine the brown sugar, granulated sugar, cinnamon and salt in a medium bowl. Drizzle in the butter and mix with a fork until just combined. Add the flour and mix with the fork until it has been evenly distributed and small to medium clumps have formed. Do not overmix or it will turn into a paste.
For the cake: Preheat the oven to 350 degrees F. Spray an 18-by-13-inch rimmed baking sheet with cooking spray. Whisk together the flour, baking powder, baking soda and salt in a medium bowl.
Beat the butter and granulated sugar in a large bowl with an electric mixer on medium-high speed until light and fluffy, about 5 minutes. Add the egg one at a time, beating well after each addition. Beat in the vanilla. Reduce the speed to low and add the flour mixture in 3 batches, alternating with the sour cream in 2 batches, starting and ending with the flour. Beat until just incorporated.
Spoon the cake batter (it will be very thick) into the prepared baking sheet and smooth with an offset spatula. Sprinkle on the crumb topping and press gently into the batter to adhere.
Bake until the cake and topping are golden brown, and a toothpick inserted into the center of the cake comes out clean, 40 to 45 minutes. Let cool completely in the baking sheet, about 1 hour. Slice and serve from the baking sheet.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dried baked goods).
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