Preheat the oven to 375°F (190°C). Line a muffin tin with paper liners.
Prepare the streusel by mixing flour, brown sugar, cinnamon, and salt. Cut in the butter until crumbly. Set aside.
Mix dry muffin ingredients: flour, baking powder, baking soda, and salt.
In another bowl, whisk melted butter, sugar, eggs, milk, and vanilla.
Combine wet and dry ingredients until just combined—do not overmix.
Fold in blueberries gently.
Fill muffin cups nearly to the top and add streusel on each.
Bake 22–25 minutes or until golden and a toothpick comes out clean.
Cool before serving.