Grease a 9-inch square baking dish with butter or cooking spray.
Prepare the Dry Ingredients:
In a medium bowl, whisk together 2 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, and 1/2 teaspoon of salt.
Cream the Butter and Sugar:
In a large bowl, use an electric mixer to cream together 3/4 cup granulated sugar and 1/4 cup softened unsalted butter until the mixture is light and fluffy.
Add the Egg and Vanilla:
Beat in 1 large egg and 1 teaspoon of vanilla extract into the creamed butter and sugar mixture until well combined.
Combine Wet and Dry Ingredients:
Gradually add the flour mixture to the butter mixture, alternating with 1/2 cup of milk. Mix until just incorporated.
Gently fold in 2 cups of fresh blueberries.
Prepare the Topping:
In a small bowl, mix together 1/2 cup of brown sugar, 1/3 cup of all-purpose flour, and 1/2 teaspoon of ground cinnamon.
Cut in 1/4 cup of chilled, diced unsalted butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Assemble the Cake:
Spread the batter evenly in the greased baking dish.
Sprinkle the topping mixture evenly over the batter.
Bake the Cake:
Place the baking dish in the preheated oven.
Bake for 35 to 40 minutes, or until a toothpick inserted in the center comes out clean and the topping is golden brown.
Cool and Serve:
Allow the Blueberry Buckle to cool slightly in the baking dish.
