Step 1

Preheat the oven to 375°.

Step 2

For the streusel: Stir together the melted butter, brown sugar, granulated sugar, and cinnamon in a medium bowl. Add the flour and salt, stirring until combined. Set aside while making the cake.

Step 3

For the buckle: Stir together the flour, baking powder, and salt in a medium bowl. Beat the butter and sugar at medium speed using a handheld mixer or stand mixer until creamy in a large bowl. Add eggs, one at a time, beating until just combined after each addition. Gradually add the flour mixture to the butter mixture, alternating with milk, beating until just combined after each addition. Stir in the lemon zest and vanilla, then gently fold in 2 cups of the blueberries.

Step 4

Spray a 9-by-9-inch baking dish with nonstick cooking spray. Spread the batter evenly in the prepared pan; sprinkle the top of the batter with the remaining 1 cup blueberries. Using your fingers, crumble the streusel mixture and sprinkle it evenly over the berries and batter.

Step 5

Bake until a tester comes out clean, 40 to 45 minutes. Let cool 20 minutes before slicing.