Preheat oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, whisk together flour, baking powder, salt, and sugar.
In another bowl, whisk milk, egg, melted butter, and vanilla extract until smooth.
Pour wet ingredients into dry ingredients. Stir gently until just combined (do not overmix).
Fold in your choice of add-ins, if using.
Scoop batter into muffin cups, filling each about ¾ full.
Bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean.
Cool on a wire rack before serving warm or storing.