Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
In another bowl, combine the melted butter, egg, milk, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the blueberries, being careful not to overmix.
In a small bowl, prepare the streusel topping by mixing together the flour, brown sugar, softened butter, and cinnamon until crumbly.
Fill each muffin cup about two-thirds full with the batter, then sprinkle the streusel topping generously over each muffin.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.