• Preheat oven to 350℉.
• Using a pastry brush (or your fingers), grease the inside of a 10” springform pan (or tube pan with a removable bottom) with 1 Tbsp softened or melted butter. Sprinkle 1 to 2 Tbsp of flour into the pan and rotate to evenly coat the bottom and sides. Tap out and discard any excess flour.

Preparing the Streusel Topping

• In a large bowl, whisk together flour, cinnamon, salt, and brown sugar until well combined.
  1 cup all-purpose flour

  ¼ tsp ground cinnamon

  ¼ tsp kosher salt

  ⅓ cup light brown sugar, tightly packed
• Add in cubed butter. Using your fingers, work the butter into the dry mixture until it becomes the consistency of wet sand and forms large crumbles. (It may seem too dry at first, but keep working at it!)
  7 Tbsp unsalted butter, chilled and cut into ½" cubes
• Add in chopped walnuts. Using your fingers, continue to mix until well combined. Set the streusel in the refrigerator to chill while preparing the cake.
  ⅓ cup walnuts, chopped

Preparing the Batter

• In a large bowl, whisk together flour, baking powder, baking soda, and salt until well combined. Set aside.
  2 cups all-purpose flour

  1 ½ tsp baking powder

  ½ tsp baking soda

  1 tsp kosher salt
• In a second large bowl, using an electric hand mixer (or a stand mixer), beat butter on high speed until it is smooth and creamy.
  ½ cup (1 stick) unsalted butter, at room temperature
• Add in sugar. Continue to beat until the mixture becomes light and fluffy.
  1 cup granulated sugar
• Add in the eggs, one at a time, and continue to mix until well combined.
  2 large eggs, at room temperature
• Add in vanilla extract and lemon zest. Continue to mix on high speed until the mixture is thick, creamy, and very pale yellow (about 2 to 3 minutes).
  1 tsp pure vanilla extract

  2 tsp lemon zest
• Reduce mixing speed to low. Into the butter/sugar/eggs mixture, add in half of the flour mixture and continue to mix until just combined. Add in buttermilk and continue to mix until just combined. Add in the remaining half of the flour mixture and continue to mix until just combined. (Be careful not to overmix!) Note: the batter will be quite thick (this is okay).
  ½ cup buttermilk, at room temperature
• Using a rubber spatula, fold in blueberries until well distributed throughout the batter.
  2 pints (4 cups) fresh blueberries, at room temperature

Assembling and Baking the Blueberry Buckle

• Spoon the batter into your prepared pan and spread it out evenly into the bottom and edges of the pan, ensuring that no air pockets form in the batter.
• Remove the streusel from the refrigerator and sprinkle it evenly over the top of the batter.
• Set the pan into the center of the oven and bake until the top is a deep golden brown and a toothpick inserted into the center comes out clean (about 60 minutes). Note: the center may sink or “buckle” slightly, even though it has finished baking.

  Pro tip: Depending on the temperature of the blueberries, the baking time can vary greatly for this recipe. If the berries are cold (or frozen) going into the batter, the cake will likely need an additional 10 to 30 minutes of baking time (I recommend checking every 5 to 10 minutes). If at any point the streusel begins to become too dark, gently set a piece of aluminum foil over the top until it is finished baking.
• Remove from the oven and allow to cool for at least 20 minutes before attempting to remove it from the pan. Serve warm, or allow to cool to room temperature. Once fully cooled, store in an airtight container (or cover with plastic wrap) at room temperature for up to 3 days.