Preheat your oven to 375°F (190°C). Grease a 9-inch square baking dish and set aside.
In a medium bowl, whisk together the flour, baking powder, and salt for the cake.
In a large bowl, cream together the granulated sugar and softened butter until light and fluffy. Beat in the egg and vanilla extract until smooth.
Gradually add the flour mixture to the butter mixture, alternating with the milk. Stir just until combined—do not overmix.
Gently fold in the blueberries, then spread the batter evenly in the prepared baking dish.
For the topping, combine the brown sugar, flour, and cinnamon in a small bowl. Cut in the chilled butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Sprinkle the topping evenly over the blueberry batter.
Bake in the preheated oven for 35–40 minutes, or until a toothpick inserted into the center comes out clean and the topping is golden brown.
Allow the buckle to cool slightly before slicing. Serve warm for the best flavor and texture.
Enjoy!
