Adapted from Cook’s Illustrated Blueberry Buckle recipe, Published 2005

Streusel:

• 1 cup packed light brown sugar
• 1 cup unbleached all-purpose flour
• 4 tbsp granulated sugar
• 1/2 tsp ground cinnamon
• pinch Kosher salt
• 8 tbsp (1 stick butter), cut into 16 pieces, softened, but still cool

Cake:

• 1 1/2 cups unbleached all-purpose flour
• 1 1/2 tsp baking powder
• 10 tbsp unsalted butter (1 1/4 sticks), softened, but still cool
• 2/3 cup granulated sugar
• 1/2 tsp Kosher salt
• the zest of 1 lemon, (zest removed by microplane grater)
• 1 1/2 tsp vanilla
• 2 eggs, room temperature
• 4 cups fresh blueberries, rinsed, dried, and picked over

For the streusel: In a standing mixer fitted with a flat beater, combine both sugars, flour, cinnamon, and salt on low speed until well combined and no large brown sugar lumps remain, about 45 seconds. Add butter and mix on low until mixture resembles wet sand and no large butter pieces remain, about 3 minutes. Transfer to a medium bowl and store in refrigerator while preparing cake.

For cake: Adjust oven rack to lower middle position and heat oven to 350 degrees.  Grease a 9×2 round cake pan and line with parchment paper round. Grease parchment paper round and dust pan with flour, knocking out excess.  
Whisk flour and baking powder in a small bowl to combine; set aside. In a standing mixer fitted with a flat beater, cream butter, sugar, salt, and lemon zest at medium-high speed until light and fluffy, about 3 minutes, stopping mixer to scrape bowl with a rubber spatula halfway through. Scrape bowl again and add vanilla. Beat at medium for about 30 seconds, and with mixer running, add eggs one at a time.  Once eggs are halfway incorporated, turn mixer off and scrape bowl with spatula.  Continue beating on medium until eggs are fully incorporated (mixture will look broken).  With mixer running on low speed, add flour mixture in thirds (this should take about 30-40 seconds), then beat until flour is almost totally incorporated (this should take about 20 seconds).  Disengage bowl from mixer then stir bowl with spatula, scraping bottom and sides of bowl, until no pockets of flour remain and batter is homogenous. Batter will be very thick.  Using rubber spatula, fold in berries until evenly distributed. 
Transfer batter into prepared pan and, using a rubber spatula, spread batter evenly to pan edges, smoothing top. Squeeze a handful of streusel topping to form a large cohesive clump, then gently break off small clumps and sprinkle streusel evenly over batter. Repeat with remaining streusel.  Bake until deep golden brown, and a toothpick comes out clean, about 55 minutes. Cool on a wire rack for 20 minutes.
Run a paring knife around sides to loosen, then place upside down plate on top of cake pan (not plate you are going to use for serving). Invert cake to remove from pan, then peel off parchment. Reinvert cake onto serving platter. Cool for at least one hour before cutting.