Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9-inch square baking dish or line it with parchment paper.

Prepare the Batter: In a mixing bowl, cream together the sugar and butter until light and fluffy. Add the egg and beat until well combined. Gradually mix in the milk.

Mix the Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Add Blueberries: Gently fold in the blueberries, being careful not to overmix. Spread the batter evenly into the prepared baking dish.

Make the Crumb Topping: In a small bowl, mix the sugar, flour, and cinnamon. Cut in the butter with a fork or pastry cutter until the mixture is crumbly.

Add the Topping: Sprinkle the crumb mixture evenly over the batter.

Bake: Bake in the preheated oven for 40–45 minutes, or until a toothpick inserted into the center comes out clean.

Cool and Serve: Allow the buckle to cool for at least 10 minutes before slicing. Serve warm or at room temperature.