Preheat your oven to 350°F (175°C). Line a 9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal, or lightly butter it. Set aside. This prevents sticking and makes cleanup easier. 5 minutes.

In a medium bowl, whisk together 2 cups all-purpose flour, 2.5 teaspoons baking powder, and 0.5 teaspoon salt until fully combined. Set aside. This mixture will be added to the batter in alternating steps. The whisking incorporates air into the flour. 2 minutes.

In a large mixing bowl, add 0.5 cup softened unsalted butter and 0.75 cup granulated sugar. Using an electric mixer on medium speed, beat for 2–3 minutes until the mixture is light, pale, and fluffy. You should see the mixture transform from thick and dense to a lighter, airier texture. Proper creaming creates a tender crumb. 3 minutes.

Add 2 large eggs to the butter-sugar mixture, one egg at a time. Beat on medium speed for about 1 minute after each addition, ensuring each egg is fully incorporated before adding the next. Scrape down the sides of the bowl as needed. Properly beaten eggs add lift and richness. 2 minutes total.

Add 1 teaspoon vanilla extract to the egg mixture and beat for 30 seconds on medium speed until combined. 1 minute.

Now alternate adding the dry flour mixture and 0.5 cup whole milk to the wet ingredients, starting with about one-third of the flour mixture. Beat on low speed for 30 seconds until just combined. Add half the milk, beat briefly, then add another third of the flour, beat, add the remaining milk, beat, then add the final flour portion. Beat just until combined after the last addition—do not overmix or the cake will be tough. Overmixing develops gluten, which can make the cake dense. 3 minutes total.

Place 2.5 cups fresh or frozen blueberries (do not thaw if frozen) in a small bowl. Sprinkle 1 tablespoon all-purpose flour over the berries and gently toss with a fork until each berry is lightly coated. The flour coating prevents the berries from sinking to the bottom during baking. 1 minute.

Gently fold the floured blueberries into the cake batter using a rubber spatula with 8–10 slow, careful strokes. Do not overmix; it's okay if the batter is not perfectly uniform. Some berry streaks are desirable. Gentle folding keeps the berries intact and prevents excessive mixing. 1 minute.

Pour the batter into the prepared 9-inch square pan and smooth the top using an offset spatula or the back of a spoon. The batter should fill the pan evenly. The batter will be thick and studded with berries. 2 minutes.

In a small bowl, combine 0.5 cup all-purpose flour, 0.5 cup packed brown sugar, 0.25 teaspoon ground cinnamon, and 0.25 teaspoon ground nutmeg. Whisk together until evenly distributed. These dry ingredients form the base of the streusel. 1 minute.

Add 0.25 cup cold unsalted butter, cut into small cubes, to the flour-sugar mixture. Using a fork or your fingertips, work the cold butter into the dry ingredients until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter remaining. Do not overwork—these butter pieces create the crunchy texture. 2 minutes.

Stir 0.5 cup chopped walnuts or pecans into the streusel mixture until evenly distributed. 1 minute.

Sprinkle the entire streusel topping evenly over the cake batter, making sure to cover it completely. Some areas may be thicker than others, which is fine. The topping should not be pressed down. 2 minutes.

Place the pan on the center oven rack (if your oven has uneven heating, position it toward the back). Bake for 45–55 minutes until the streusel topping is deep golden brown and a toothpick inserted into the cake portion (avoiding the berries) comes out clean or with just a few moist crumbs. The cake should spring back slightly when lightly pressed. Do not insert the toothpick into a berry, as this will give a false wet reading. Baking time varies by oven. Start checking at 45 minutes. 50 minutes average.

Checkpoint: You are ready to move on when the streusel is golden brown, the cake springs back when pressed, and a toothpick inserted into the cake (not a berry) comes out clean or nearly clean.

Remove the pan from the oven and place it on a wire cooling rack or heat-safe surface. Let the buckle cool in the pan for 15–20 minutes at room temperature. The residual heat will continue to set the cake while the streusel crisps up. The cake will be easier to slice when slightly warm or at room temperature. 18 minutes.

Once cooled slightly, you may lift the entire buckle out of the pan using the parchment paper overhang, or simply serve directly from the pan. The buckle is best served warm or at room temperature, with or without accompaniments.