Prepare for Baking:
Preheat your oven to 375°F (190°C). Grease a 9-inch baking pan or line it with parchment paper.
Make the Cake Batter:
In a bowl, whisk together the flour, baking powder, and salt. In a separate large bowl, cream the butter and sugar until light and fluffy. Beat in the egg and vanilla extract. Gradually add the flour mixture, alternating with milk, starting and ending with the dry ingredients. Fold in the blueberries gently to avoid crushing them.
Prepare the Crumb Topping:
In a small bowl, mix the sugar, flour, and cinnamon. Cut in the cold butter with a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Assemble the Buckle:
Spread the batter evenly into the prepared pan. Sprinkle the crumb topping over the batter, covering it completely.
Bake:
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.