New Jersey Crumb Cake

Using a fork or your fingers, mix 3 cups plus 2 Tbsp. (391 g) all-purpose flour, 1 cup (packed; 200 g) dark brown sugar, ¼ cup (50 g) granulated sugar, 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, and 2 tsp. ground cinnamon in a medium bowl to combine. Add 1¼ cups (2½ sticks) unsalted butter, melted, slightly cooled, and mix until large crumbs form and no dry streaks of flour remain. Let crumb topping sit while you make the cake batter, at least 10 minutes.

Place a rack in middle of oven; preheat to 350°. Lightly coat a 9x9" metal cake pan with nonstick vegetable oil spray and line pan with parchment paper, leaving at least a 2" overhang on 2 sides. Whisk 1 cup (125 g) all-purpose flour, 1 tsp. baking powder, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt together in a medium bowl. Whisk together 1 large egg, room temperature, ¾ cup (150 g) granulated sugar, ½ cup sour cream, room temperature, ⅓ cup vegetable oil, and 1 tsp. vanilla extract in a second medium bowl until smooth. Add to dry ingredients and gently whisk until almost completely combined. Using a rubber spatula, fold batter a few times to combine completely (it’s okay if there are a few lumps). Scrape batter into prepared pan and smooth into an even layer.

Using your hand, squeeze a palm-size piece of crumb topping together. Starting from the sides of the pan, crumble mixture in your hand over batter, aiming to create about ¼" pieces. Repeat process, working your way toward the center, until you’ve used about two thirds of topping. (The surface of the cake should be completely covered with crumbs.) Crumble remaining topping over entire cake, aiming for larger, about ½" pieces. (It will seem like too much crumb, but that’s the point.)

Bake until cake starts to pull away from edges of pan, a tester inserted into the center comes out clean, and topping is set a little but still looks moist in places, 36–42 minutes. Transfer pan to a wire rack and let cake cool in pan, about 2 hours.

Using parchment overhang, remove cake from pan and transfer to a cutting board. Dust generously with powdered sugar, then slice into a 4x3 grid for the classic rectangular slices you’d find in a Jersey bakery or deli, or a 4x4 grid for more modest squares. (If you’d like your topping to be extra-tender, wrap cake in plastic wrap and let sit 8 hours before serving.) Do Ahead: Cake can be baked 4 days ahead. Store tightly wrapped at room temperature.