Preheat the oven to 350°F (175°C) and grease an 8x8-inch baking dish.
Mix flour, granulated sugar, brown sugar, baking powder, baking soda, and salt in a bowl.
In another bowl, whisk melted butter, buttermilk, egg, and vanilla extract.
Gradually fold wet ingredients into the dry mixture until combined.
Gently fold in the blueberries, making sure not to crush them.
Pour the batter into the prepared baking dish and smooth the top.
For the streusel, combine brown sugar, flour, and cinnamon in a bowl.
Cut in cold butter until the mixture forms crumbs, then sprinkle evenly over the batter.
Bake for 35 minutes or until a toothpick comes out clean from the center.
Cool slightly before cutting into squares, or eat directly with a fork—no judgment.