Preheat Oven:

Set your oven to 350°F (175°C). Grease an 8-inch square baking dish or a deep-dish pie plate.

Make the Batter:

In a large bowl, cream the butter and sugar until fluffy. Beat in the egg, vanilla, and lemon zest until combined.

Combine Dry Ingredients:

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry mix to the wet ingredients, alternating with the milk. Mix until just combined—don’t overmix.

Add Blueberries:

Gently fold in the blueberries with a spatula to avoid crushing them.

Spread Batter:

Pour the batter into the prepared baking dish and smooth out the top.

Make the Crumble:

In a medium bowl, use a fork or pastry cutter to combine butter, sugar, flour, cinnamon, and a pinch of salt until it resembles coarse crumbs.

Top and Bake:

Sprinkle the crumb topping evenly over the batter. Bake for 45–50 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.

Cool and Serve:

Let the buckle cool for at least 15 minutes before slicing. Serve warm or at room temperature.