Preheat oven to 350°F and line an 8x8 square cake pan with parchment paper, or coat lightly with nonstick spray.
In a large bowl, combine flour, granulated sugar, baking powder, salt, and cinnamon.
1 cup all-purpose flour, 1/3 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon salt, 1/4 teaspoon ground cinnamon
Whisk together milk, egg, and vanilla, then pour into dry ingredients and stir to combine.
1/2 cup milk, 1 egg, 1 teaspoon pure vanilla extract
Fold in blueberries, then pour batter into prepared pan.
1 cup fresh blueberries
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 over the top of the batter.
1/3 cup all-purpose flour, 2 tablespoons brown sugar, 2 tablespoons granulated sugar, Pinch salt, 2 tablespoons cold butter
Bake for 30-35 minutes or until golden brown and a toothpick inserted into the center comes out clean.