Preheat oven to 350 degrees F. Liberally grease a 13x9" baking pan with cooking spray and set aside.
For the crumble, in a medium bowl, stir together the brown sugar, flour, cinnamon, and melted butter together until large clumps form and butter has completely combined with the mixture. Set aside.
For the batter, in the bowl of a stand mixer, beat together the butter and sugar on medium-high speed until fluffy and light, about 3 minutes. Add in the eggs, one at a time, beating until just combined. Add in the vanilla and sour cream, then stir in the flour, baking powder, baking soda, and salt. Be careful not to overmix.
Spread the batter into the prepared pan evenly, then, using your hands, clump and crumble the crumb topping over the mixture, taking care to cover the batter below. Bake for approx. 30-35 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Cool 10-15 minutes before cutting into squares, then dust with powdered sugar if using.