Preparation and Baking

• Combine the brown sugar, flour, cinnamon, and salt in a small bowl, then work through any brown sugar clumps with your fingers. Rub in the cold butter chunks until the mixture forms uneven crumbs, then refrigerate it while you make the batter.
• Position a rack in the center of the oven and heat it to 375°F (190°C), then grease a 9-inch (23-cm) square pan with butter or nonstick cooking spray. The center rack encourages even browning, while the coated pan helps the finished cake release cleanly.
• Whisk the flour, baking powder, and salt together in a medium bowl. Mix until the baking powder is evenly dispersed and no streaks of salt remain.
• Beat the softened butter with the granulated sugar in an electric mixer for about 2 minutes, then beat in the eggs one at a time, scraping the bowl between additions. Continue with the vanilla and lemon zest once the mixture looks pale, billowy, and noticeably lighter in texture.
• Add the flour mixture and milk in alternating additions on low speed, stopping as soon as the batter comes together, then fold in the blueberries with a spatula. The finished batter should be thick enough to fall in slow ribbons and should hold whole berries without crushing their skins.
• Spread the batter evenly in the prepared pan and scatter the chilled streusel across the surface. Bake for 40 to 45 minutes, until the edges are golden, the top smells warmly of cinnamon, and a cake tester from the center comes out clean.
• Cool the cake in its pan on a wire rack for about 20 minutes before slicing. The crumb finishes setting as it cools, so the slices hold together while the berries remain juicy.
• Serve the cake on the day it is baked for the crispest topping. Wrap any remaining cake in foil and keep it protected from humidity so the streusel stays as textured as possible.