Step 1

Cut butter into cubes and prepare the baking pan. Cut 6 tablespoons unsalted butter into small cubes, then place in a stand mixer and let sit at room temperature to soften, about 15 minutes. (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 375°F. Coat an 8-inch square baking pan with cooking spray, then line with a parchment paper sling so that it hangs off two sides. Prepare the streusel.

Step 2

Melt the butter for the streusel. Microwave 4 tablespoons unsalted butter in a small microwave-safe bowl in 10-second intervals until melted, about 40 seconds total.

Step 3

Zest the lemon and rub into the sugar. Finely grate the zest of 1 medium lemon (about 1 tablespoon). Reserve the zested lemon for the icing. Place the zest and 1/2 cup granulated sugar in a medium bowl, and rub together with your fingers until very fragrant and the consistency of wet sand.

Step 4

Make the streusel. Add 1/2 cup cake flour and 1/4 teaspoon kosher salt and stir with a spatula until combined. Add the melted butter and stir until no flour remains and the mixture is crumbly. Freeze until chilled, 10 to 15 minutes.

Step 5

Mix the dry ingredients. Place 2 1/4 cups cake flour, 2 teaspoons baking powder, and 1/2 teaspoon kosher salt in a medium bowl and whisk to combine.

Step 6

Cream the butter and sugar. Add 3/4 cup granulated sugar to the stand mixer. Beat with the paddle attachment on medium speed until lightened in color, about 4 minutes. Scrape down the sides of the bowl.

Step 7

Make the buckle batter. With the mixer on low speed, beat in 2 large eggs one at a time, waiting until the first is completely incorporated before adding the second. Add 2 teaspoons vanilla extract and beat until combined. Add half of the flour mixture and beat until mostly incorporated. Add 1/2 cup whole or 2% milk and beat until just combined. Scrape down the sides and bottom of the bowl. Return the mixer to low speed, add the remaining flour mixture, and mix until just combined. (The dough will be thick, so if using a hand mixer, mix in the remaining flour mixture by hand with a sturdy spatula.)

Step 8

Fold in the blueberries. Add 1 cup of the blueberries and fold in by hand with a rubber spatula.

Step 9

Transfer to pan and add blueberries.Transfer the batter to the prepared baking pan and smooth the top. Sprinkle the remaining 1 cup blueberries evenly onto the batter.

Step 10

Top with streusel. Crumble the chilled streusel in large pieces evenly over the top.

Step 11

Bake the buckle. Bake until the cake is lightly browned and a toothpick inserted into the center comes out clean, 40 to 45 minutes.

Step 12

Cool the buckle. Place the pan on a wire rack and cool for 15 minutes. Grasping the parchment paper, remove the cake from the pan to the wire rack and let cool completely. Prepare the glaze, if desired.

Step 13

Make the lemon glaze. Juice the reserved lemon (about 2 tablespoons). Place 1/2 cup powdered sugar in a small bowl, add 1 tablespoon of the lemon juice, and stir with a spatula until smooth. If needed, stir in 1 to 2 teaspoons more lemon juice until the glaze is a pourable consistency.

Step 14

Drizzle glaze over buckle and serve. Drizzle the cooled blueberry buckle with the lemon glaze. Let sit until the glaze sets, about 15 minutes. Cut into squares and serve.