Preheat the oven to 350 degrees Fahrenheit and grease a 9x9-inch baking pan.

Whisk together the 2 1/2 cups of flour, baking powder, salt, and ground cinnamon in a medium bowl.

Toss the blueberries with 1 tablespoon of flour in a small bowl and set aside.

In a large bowl, cream together the butter and granulated sugar until light and fluffy.

Beat in the sour cream, then add the egg and mix until smooth.

Stir in the milk and vanilla extract until well combined.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Gently fold in the flour-coated blueberries.

Spread the batter evenly into the prepared pan.

In a separate bowl, combine the topping flour, brown sugar, and cinnamon.

Cut in the cold butter until the mixture resembles coarse crumbs.

Sprinkle the topping evenly over the batter.

Bake for 45 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan before slicing and serving.