Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners for easy removal.

In a mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until everything is evenly mixed.

In another bowl, combine the melted butter, buttermilk, eggs, and vanilla extract. Whisk until smooth.

Pour the wet ingredients into the dry ingredients. Stir gently until just combined, remembering that a few lumps are perfectly fine.

Gently fold in the fresh blueberries, being careful not to break them.

For the streusel, mix the brown sugar, flour, cold butter, and cinnamon in a small bowl until it resembles coarse crumbs.

Fill each muffin cup about 2/3 full with the batter, then sprinkle the streusel topping on top.

Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.