Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, mix together the flour, sugar, baking powder, and salt.
In another bowl, whisk together the melted butter, milk, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Gently fold in the blueberries, if using.
Divide the batter evenly among the muffin cups.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.