• Preheat oven to 325°F. Line an 8×8-inch baking dish with aluminum foil, including up the sides, leaving extra foil hanging over the edges. Spray with nonstick cooking spray. Set aside.

Crumb Topping

• To a medium bowl, add butter, granulated sugar, brown sugar, cinnamon, and salt. Stir to combine.
• Add the cake flour. Stir until the mixture is completely combined. Be careful not to overmix or it can become like a paste.

Cake

• To the bowl of a stand mixer fitted with paddle attachment, add flour, sugar, salt, and baking soda. Mix on low speed to combine.
• Add butter pieces, one at a time, mixing until there are no visible butter pieces and it has a very fine crumb, about 1 minute.
• Add buttermilk, whole egg, egg yolk, lemon zest, lemon juice, and vanilla. Beat on medium speed until light and fluffy, about 1 minute, stopping to scrape the bowl as needed.
• Pour batter evenly into the prepared pan, spreading with a spatula if needed.
• Sprinkle the blueberries evenly on top of the batter.
• Using your hands or a fork, break up the topping into pieces. Spread the topping in an even layer on top of the blueberries.
• Bake for 45-50 minutes, or until crumbs are golden and a toothpick inserted into the center of the cake comes out clean.
• Let the cake cool in the pan on a wire rack for 30 minutes. Then, using the excess aluminum foil, lift the cake out of the pan to continue cooling.
• Dust with confectioners’ sugar just before serving.