Preheat your oven to 375°F (190°C). Grease a 9-inch square baking dish with butter or cooking spray to prevent sticking.

In a medium bowl, whisk together flour, baking powder, and salt. Set aside.

In a large mixing bowl, beat granulated sugar with softened butter until light and fluffy, about 3-5 minutes. Use an electric mixer for best results.

Beat in the egg and vanilla extract until the mixture is smooth and evenly combined.

Gradually add the dry ingredients to the creamed mixture, alternating with milk. Begin and end with the dry ingredients. Stir just until combined.

Gently fold in the blueberries using a spatula, being careful not to crush them.

In a small bowl, combine brown sugar, flour, and cinnamon. Cut in chilled butter using a pastry cutter or two knives until the mixture resembles coarse crumbs.

Spread the batter evenly into the prepared baking dish. Sprinkle the crumb topping over the batter. Bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean and the topping is golden brown.

Let the buckle cool in the pan on a wire rack for 10-15 minutes. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.