Step 1

Cut 6 tablespoons unsalted butter into small cubes, then place in a stand mixer and let sit at room temperature to soften. (Alternatively, place in a large bowl if using an electric hand mixer.) Meanwhile, arrange a rack in the middle of the oven and heat the oven to 350°F. Line an 8-inch square baking pan with two overlapping sheets of parchment paper so that it hangs off all four sides and forms a sling. Coat the corners of the pan not covered with the parchment with cooking spray. Prepare the streusel.

Step 2

Make the streusel. Place 4 tablespoons unsalted butter in a medium microwave-safe bowl and microwave in 15-second bursts until completely melted, about 45 seconds total. (Alternatively, melt in a small saucepan over medium-low heat.)

Step 3

Finely grate the zest of 1 medium lemon (about 1 tablespoon). Reserve the zested lemon for the cake. Add the zest, 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup packed light brown sugar, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon kosher salt to the melted butter and stir with a fork until evenly moistened and a variety of crumb sizes form. Do not mix into a paste of uniform consistency. Refrigerate until the butter firms up, at least 10 minutes. Meanwhile, make the cake.

Step 4

Make the cake. Place 1 1/2 cups all-purpose flour, 2 teaspoons baking powder, 1/2 teaspoon kosher salt, and 1/4 teaspoon baking soda in a medium bowl and whisk to combine.

Step 5

Place 1/4 cup whole milk, 1 large egg, 3/4 cup sour cream, 1 tablespoon vanilla extract, and the juice of the zested lemon (about 2 tablespoons) in a large liquid measuring cup or bowl and whisk to combine.

Step 6

Add 1/2 cup granulated sugar and 1/2 cup packed light brown sugar to the butter in the stand mixer. Beat with the paddle attachment on medium speed until lightened in color, about 7 minutes. With the mixer on low speed, add half of the flour mixture and beat until mostly incorporated. Add the milk mixture and beat until just combined. Scrape down the sides and bottom of the bowl.

Step 7

Return the mixer to low speed, add the remaining flour mixture, and beat until just combined. Stop the mixer, add 1 cup of the blueberries, and fold in by hand with a rubber spatula.

Step 8

Transfer the batter to the prepared baking pan and smooth the top. Sprinkle the remaining 1/2 cup blueberries evenly onto the batter. Crumble the chilled streusel evenly over the top.

Step 9

Bake until the cake is lightly browned and a toothpick inserted into the center comes out clean, 50 to 60 minutes. Place the pan on a wire rack and cool for 15 minutes. Grasping the excess parchment, lift the cake out of the baking dish and set it on the rack. Let cool completely. Transfer the cake to a cutting board, cut into squares, and serve.