- 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.