To make the streusel:

In the bowl of a stand mixer fitted with the paddle attachment, beat the flour, both sugar, cinnamon, and salt on low to combine and break up any brown sugar lumps. With the mixer still on low, add the butter and beat until it is completely incorporated into the dry ingredients, about 2-3 minutes – the mixture will resemble wet sand. Transfer the streusel to another bowl and set aside.

For the equipment:

Preheat oven to 350 F with a rack in the lower third of the oven. Line the bottom of a 9-inch round or springform pan with a round of parchment, then spray the parchment and the pan with nonstick cooking spray. Sprinkle with flour and tap out any excess.

For the buckle:

In a small bowl, whisk the flour and baking powder together then set aside. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar, salt and lemon zest at medium speed until light and fluffy, about 3-4 minutes, stopping to scrape down the sides of the bowl as necessary. Beat in the vanilla and lemon oil if using. Add eggs, one at a time, beating well after each addition.  With the mixer on low, gradually add the flour mixture, beating until just about incorporated. Remove the bowl from the mixer and use a rubber spatula to finish stirring – the batter will be very thick. Gently fold in the blueberries.

Transfer the batter to the prepared pan; spread in an even layer. To top with the streusel – pick up a handful of streusel and squeeze to form a clump.  Break this large clump into smaller pieces and sprinkle over the batter. Continue until you’ve used all of the streusel.

Bake for about 55 minutes, or until the streusel is lightly golden brown and the cake edges have slightly pulled away from the pan and are golden. The top will be a bit wobbly if pressed and the toothpick test is unreliable. Transfer pan to a wire rack and cool for 15-20 minutes. Run a thin knife around the edges of the cake then invert cake onto the rack. Remove the parchment then turn the cake streusel side up and let sit until only slightly warm or completely cool on the wire rack.

Top with powdered sugar if desired and transfer to serving platter.

Note:

• My variation uses for the streusel, 1/2 cup flour, a heaping 1/2 cup white sugar, 5 tablespoons of softened butter, 1/4 teaspoon salt. For the buckle, I omit the vanilla and use 4 to 5 drops of lemon essential oil or 1/2 teaspoon lemon extract.
• Other fruit may be substituted for all or part of the blueberries. If fruit is very juicy, toss in a little flour.