Heat the oven to 375°F (190°C). Grease an 8-inch square baking pan, and line the bottom with parchment if you want especially easy serving.

Cream the softened butter and granulated sugar until the mixture looks light and smooth. This creates the foundation for a tender cake crumb.

Beat in the egg and vanilla extract until evenly incorporated. Scrape down the bowl so the butter mixture is combined before adding the dry ingredients.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Mixing these ingredients separately helps distribute the leavening evenly.

Add the dry ingredients to the butter mixture alternately with the milk, mixing just until combined. Stop as soon as the flour disappears because overmixing can make the buckle dense.

Gently fold in the blueberries, taking care not to crush them. Spread the thick batter evenly into the prepared pan.

Make the streusel by combining the flour, brown sugar, and cinnamon. Cut or rub in the softened butter until coarse crumbs form.

Sprinkle the streusel evenly over the blueberry batter. Cover the surface from edge to edge for a buttery, cinnamon-scented topping in every serving.

Bake for approximately 35 to 45 minutes, beginning to check at 35 minutes. The top should be golden, and a toothpick inserted into the cake portion should come out clean or with a few moist crumbs.

Cool the buckle in the pan for at least 15 minutes before cutting. This brief rest helps the cake set while keeping the crumb soft and the berry juices pleasantly jammy.