Ingredients:

  • 1/2 cup unsalted butter, melted
  • 3/4 cup all-purpose flour
  • 1/2 cup granulated sugar
  • 1 teaspoon ground cinnamon
  • 1/4 teaspoon salt
  • 1/2 cup unsalted butter, softened
  • 3/4 cup granulated sugar
  • 1 large egg, room temperature
  • 1.5 teaspoons vanilla extract
  • 1/2 cup sour cream
  • 2 cups all-purpose flour
  • 2 teaspoons baking powder
  • 1/2 teaspoon kosher salt
  • 2.5 cups fresh blueberries
  • 1 tablespoon lemon zest

Instructions:

  1. Preheat your oven to 350°F (175°C). Grease a 9x9-inch square baking pan and line with parchment paper.
  2. In a medium bowl, whisk together 3/4 cup flour, 1/2 cup sugar, cinnamon, and 1/4 tsp salt. Stir in 1/2 cup melted butter with a fork until moist clumps form; set aside.
  3. In a large bowl, cream 1/2 cup softened butter and 3/4 cup sugar together until light and fluffy, about 3 minutes.
  4. Beat in the egg and vanilla extract, then fold in the sour cream until combined.
  5. Whisk together 2 cups flour, baking powder, and 1/2 tsp salt. Gradually add to the wet ingredients, mixing on low speed until just combined.
  6. Gently fold in the blueberries and lemon zest by hand using a rubber spatula.
  7. Spread the thick batter into the prepared pan. Sprinkle the prepared streusel topping evenly over the surface.
  8. Bake for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean and the topping is golden brown.
  9. Allow the buckle to cool in the pan for 20 minutes before slicing and serving.