• Preheat the oven to 350°F. Butter and flour an 9" square cake pan. Cut an 8"x16" piece of parchment paper and line the pan in one direction, leaving a short overhand on each side. Butter and flour the parchment. (see note)

Crumb Topping

• Combine 6 3/4 oz Cake flour, 6 oz brown sugar, 1/4 teaspoon table salt, 2 teaspoons cinnamon and 1/2 teaspoon lemon zest. Mix on low speed until completely combined. With the mixer running, add 4 oz unsalted butter and continue to mix until the topping looks like sand. Remove from the mixer and transfer to a small bowl by squeezing handfuls to create large clumps. Set aside while you mix the batter.

Batter

• Combine 1 large egg and 1 egg yolk, 1 teaspoon vanilla extract and 1/4 cup of the buttermilk in a small bowl and set aside.
• Sift together 6 3/4 oz Cake flour, 6 oz granulated sugar, 1 1/2 teaspoons baking powder and 1/4 teaspoon salt. add 1 teaspoon lemon zest. With the mixer running on low speed, add 4 oz unsalted butter and the remaining 1/4 cup of buttermilk. Mix until completely combined. Scrape the bowl and beater. Mix on medium speed for 2-3 minutes to lighten the batter.
• Scrape the bowl and add the egg mixture in two batches, scraping between additions. By hand, fold about 2/3 of the blueberries into the batter. Spread the batter into the prepared pan.
• Sprinkle 1/2 the remaining blueberries over the top of the batter. Sprinkle 1/2 the crumb topping over the batter, breaking the topping into pea to marble sized clumps. Sprinkle the rest of blueberries and then the remaining crumb topping.
• Bake until a toothpick inserted in the center comes out clean, about 45 minutes. Cool at least 30 minutes in the pan. Use the parchment to carefully lift the cake out of the pan.
• Cool completely on a wire rack