for the Streusel
1/2 cup all-purpose flour
1/2 cup packed light brown sugar
2 tablespoons granulated sugar
1/4 teaspoon ground cinnamon
4 tablespoons (1/2 stick) unsalted butter, cut into 8 pieces, softened
for the Cake
1 1/2 cups all-purpose flour
1 1/2 teaspoons baking powder
10 tablespoons unsalted butter, softened
2/3 cup granulated sugar
1/2 teaspoon salt
1/2 teaspoon lemon zest
1 1/2 teaspoons vanilla extract
2 large eggs, at room temperature
2 1/2 cups blueberries (fresh or frozen)
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.
Preheat oven to 350F. Line the bottom of a 9-inch round pan with a round of parchment, then spray the parchment and the pan with nonstick cooking spray with flour.
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. Add the eggs, on 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 golden brown and a toothpick inserted in the center comes out clean. Transfer pan to a wire rack and cool for 15-20 minutes. Run a thin knife around the edges of the cake then invert it onto the rack. Remove the parchment then turn the cake streusel side up and let cool on the wire rack.