Preheat oven to 375°F (190°C) and line a muffin tin with paper liners.
Whisk flour, sugar, baking powder, baking soda, and salt in a bowl.
In another bowl, whisk melted butter, eggs, buttermilk, and vanilla.
Stir wet ingredients into dry ingredients just until combined.
Gently fold in blueberries.
Mix streusel ingredients by cutting butter into flour, brown sugar, and cinnamon until crumbly.
Fill muffin cups 3/4 full and sprinkle streusel generously on top.
Bake 18–22 minutes until golden and a toothpick comes out clean.
Cool slightly before serving.