Preheat oven to 375°F (190°C).
Line muffin tin with paper liners.
Mix flour, sugar, baking powder, salt together.
Whisk melted butter, egg, milk in bowl.
Pour wet ingredients into dry ingredients.
Fold gently until just combined.
Toss blueberries with extra flour lightly.
Fold blueberries into batter carefully.
Mix streusel ingredients with fork.
Fill muffin cups 2/3 full.
Sprinkle streusel topping over each muffin.
Bake for 20-25 minutes until golden.
Cool in pan for 5 minutes.
Transfer to wire rack completely.