1. In a small bowl, combine 0.5 cup all-purpose flour, 0.33 cup brown sugar, 0.25 cup granulated sugar, 1 teaspoon cinnamon, and a pinch of salt. Add 0.25 cup (half of the butter, cold and cubed) and mix until coarse crumbs form. Set aside.

2. Preheat oven to 350°F (175°C). Grease and flour a 9-inch square baking pan.

3. In a medium bowl, whisk together 2 cups all-purpose flour, 2 teaspoons baking powder, 0.5 teaspoon baking soda, and 0.25 teaspoon salt.

4. In a large bowl, beat 0.25 cup unsalted butter (remaining half), and 0.75 cup granulated sugar until fluffy. Beat in 2 eggs, one at a time. Mix in 1 teaspoon vanilla extract and 1 teaspoon lemon zest, then add 1 cup sour cream and blend until smooth.

5. Gently fold dry ingredients into wet until just combined. Do not overmix.

6. Toss 1.5 cups blueberries with 1 tablespoon flour, then gently fold into batter.

7. Spread half the batter into the pan. Sprinkle half the streusel over. Spread remaining batter, then top with remaining streusel.

8. Bake for 45–50 minutes, until a toothpick inserted in center comes out clean.

9. Cool in pan before slicing into 12 squares.