Preheat your oven to 350°F and grease a 9x13 inch baking dish (or 8x8 if you prefer a thicker buckle). While the oven heats, prepare the streusel topping by combining the flour, brown sugar, sugar, cinnamon, and nutmeg in a small bowl. Cut the 5 tablespoons of cold butter into small cubes and work it into the dry mixture with your fingertips until it resembles coarse breadcrumbs with some pea-sized butter pieces remaining—this creates the signature crumbly texture. Set the topping aside; I like to keep it at room temperature so it stays loose and doesn't clump together when sprinkled.

In a large bowl, beat together the 3/4 cup sugar and 1/4 cup room-temperature butter until light and fluffy, about 2-3 minutes. This creaming process incorporates air and creates a tender crumb in the final cake. Add the egg and beat well, then mix in the lemon zest and vanilla extract until fully combined. The batter should look pale and slightly increased in volume.

In a separate bowl, whisk together the 1 1/2 cups flour, baking powder, and salt. Alternate adding the dry ingredient mixture and the milk to the creamed mixture from Step 2, beginning and ending with the dry ingredients—this prevents overmixing and ensures a tender cake. Stir just until combined; a few small lumps are fine and actually preferable to an overmixed batter.

Toss the 2 cups of blueberries with the 1 tablespoon of flour—this light coating helps prevent them from sinking to the bottom during baking. Gently fold the floured blueberries into the batter from Step 3 until just combined, being careful not to crush the berries. Pour the batter into your prepared baking dish and spread it into an even layer.

Sprinkle the streusel topping from Step 1 evenly over the batter, breaking up any clumps as you go. Bake in your preheated 350°F oven for 40-45 minutes, until the top is golden brown and a toothpick inserted into the cake (not through a berry) comes out clean or with just a few moist crumbs. The buckle should have a slight dome and the streusel should be crispy and set.