Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, combine the melted butter, milk, eggs, and vanilla extract.
Pour the wet ingredients into the dry ingredients, stirring until just combined.
Gently fold in your chosen mix-ins.
Evenly distribute the batter into the prepared muffin tin.
Bake in the preheated oven for 18-20 minutes.
Remove from the oven and let cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely.
Enjoy your freshly baked muffins!