Preparation
1. Preheat your oven to 375°F (190°C) and grease a 9-inch square baking dish.
2. For the cake, in a medium bowl, whisk together the 2 cups of flour, baking powder, and salt.
3. In another large bowl, cream together the 3/4 cup granulated sugar and 1/4 cup softened butter until light and fluffy. Beat in the egg and vanilla extract.
4. Gradually add the flour mixture to the butter mixture, alternating with the milk, and mix just until incorporated. Gently fold in the blueberries and spread the batter evenly in the prepared baking dish.
5. For the topping, combine the brown sugar, 1/3 cup flour, and cinnamon in a bowl. Cut in the chilled butter with a pastry blender or your fingers until the mixture resembles coarse crumbs. Sprinkle evenly over the batter in the dish.
6. Bake for 35 to 40 minutes, until a toothpick inserted in the center comes out clean and the topping is golden brown.
7. Allow the buckle to cool slightly before slicing and serving warm.
