Preheat the oven to 375°F (190°C) and grease or line a 12-cup muffin tin.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate medium bowl, whisk together the melted butter, milk, egg, and vanilla extract.
Pour the wet ingredients into the dry ingredients and fold gently until just combined; do not overmix.
Gently fold in the mixed berries until evenly distributed.
Distribute the batter evenly among the 12 muffin cups.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.