For the Streusel: In a small bowl, blend the flour, both sugars, and cinnamon. With a fork or fingertips, incorporate the softened butter until the mixture forms coarse crumbs. Refrigerate until needed.

For the Buckle: Begin by preheating the oven to 375°F and lightly greasing an 8×8-inch baking pan with baking spray. In a separate medium bowl, whisk the flour, baking powder, and salt. In a large bowl or stand mixer fitted with a paddle attachment, cream the butter, sugar, and lemon zest on medium speed for about 2 to 3 minutes until pale and fluffy. Incorporate the eggs one at a time, pausing to scrape down the sides as needed, then blend in the vanilla extract. With the mixer on low, add one-third of the dry ingredients, then pour in half the milk, mixing just until combined. Alternate adding the remaining dry ingredients and milk, finishing with the flour mixture. Gently fold in 2 cups of the blueberries. Evenly spread the batter into the prepared pan, scatter the chilled streusel over the top, and dot with the remaining ½ cup of blueberries. Bake for 50 to 55 minutes, loosely tenting with foil after 25 minutes if needed, until the surface is golden and a toothpick inserted in the center emerges clean. Allow the buckle to cool for at least 20 minutes before slicing to ensure clean cuts.