Step 1

Whisk 1 1/2 cups cake flour, 2/3 cup packed dark brown sugar, 2/3 cup granulated sugar, 1 1/2 teaspoons ground cinnamon, and 1/4 teaspoon kosher salt together in a medium bowl.

Step 2

Add 1 stick melted unsalted butter. Stir with your hands or a flexible spatula until crumbly and it holds together when squeezed.

Step 3

Heat the oven to 350ºF. Meanwhile, coat a 9x13-inch baking dish cooking spray. Line the bottom and two long sides with a sheet of parchment paper long to extend over the sides by at least 2 inches to form a sling.

Step 4

Whisk 3 cups cake flour, 1 1/2 cups granulated sugar, 1 1/2 teaspoons baking soda, and 1 1/2 teaspoons baking powder together in a large bowl.

Step 5

Whisk 1 1/2 cups sour cream, 1/4 cup whole milk, 3 large eggs, 2 large egg yolks, 12 tablespoons melted and cooled unsalted butter, 1 tablespoon vanilla extract, and 3/4 teaspoon kosher salt together in a second large bowl until combined and smooth.

Step 6

Pour into the flour mixture and stir with a flexible spatula until just combined (some small lumps are okay).

Step 7

Transfer the batter to the baking dish and smooth out the top. Using your hands, crumble the topping into rough pieces in varying sizes no bigger than a blueberry evenly over the cake. Gently press the crumbs into the batter.

Step 8

Bake, rotating the baking dish halfway through, until a tester inserted into the center comes out clean, 40 to 60 minutes total.

Step 9

Let cool for at least 1 hour before serving. Serve out of the baking dish. Alternatively, run a knife along the edges of the cake not touching parchment to loosen, then grasp the excess parchment and lift the cake slab onto a cutting board before cutting. Lightly dust with powdered sugar if desired.