Step 1

The batter will be extremely thick and heavy, and some effort will be required to spread it into the prepared cake pan. This buckle is best made with fresh blueberries, not frozen ones, which are too moist. Leftovers can be wrapped and stored at room temperature for up to two days.

Step 2

FOR THE STREUSEL : Combine flour, brown sugar, granulated sugar, cinnamon, and salt and stir with fork until no lumps remain. Stir in melted butter until mixture resembles wet sand.

Step 3

FOR THE CAKE : Adjust oven rack to lower-middle position and heat oven to 350 degrees. Spray 8½ by 4½-inch loaf pan with vegetable oil spray. Line pan with 10½ by 8½-inch sheet of parchment, smoothing it up long sides of pan. Spray parchment.

Step 4

Whisk flour and baking powder in small bowl to combine; set aside. Combine melted butter, sugar, egg, vanilla, salt, and lemon zest in medium bowl and whisk until smooth. Add flour mixture and stir with spatula until smooth. Gently fold in blueberries until evenly distributed.

Step 5

Transfer batter to prepared pan; with rubber spatula, using a pushing motion, spread batter evenly to pan edges and smooth surface. Squeeze handful of streusel in your hand to form large cohesive clump; break up clump with your fingers and sprinkle evenly over batter. Repeat with remaining streusel. Bake until deep golden brown and toothpick inserted into center of cake comes out clean, about 50 minutes. Let cool on wire rack for 20 minutes (cake will fall slightly as it cools).

Step 6

Run paring knife along short sides of cake to loosen. Grasping parchment, gently lift cake from pan. Peel parchment from sides of cake and let cool until just warm or to room temperature, at least 30 minutes. Peel parchment from bottom of cake. Slice and serve.