Preheat oven to 350°F and line a 9x9-inch square cake pan with parchment paper, or coat lightly with nonstick spray.
In a bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
1 1/2 cups all-purpose flour, 1 teaspoons baking powder, 1/2 teaspoon fine sea salt, 1/2 teaspoon baking soda
In a large bowl, cream together softened butter and granulated sugar until fluffy, then beat in sour cream, eggs, and vanilla until smooth. Add the dry ingredients, mixing until just incorporated.
8 tablespoons unsalted butter, 1 cup sour cream, 2 large eggs, 1 teaspoon pure vanilla extract, 2/3 cup granulated sugar
Toss the blueberries with a bit of flour. Fold the blueberries into the batter, then spread into the prepared pan.
To make the streusel, in a small bowl combine flour, brown sugar, granulated sugar, and salt. Cut in butter using a fork until mixture forms large crumbs. Scatter evenly over the top of the batter.
1/3 cup all-purpose flour, 2 tablespoons brown sugar, 2 tablespoons granulated sugar, Pinch fine sea salt, 2 tablespoons cold butter
Bake for 30-35 minutes or until golden brown and a toothpick inserted into the center comes out clean.