Preheat oven to 375°F (190°C) and grease a mini muffin tin or line with paper liners.
In a large bowl, whisk together flour, sugar, baking powder, and salt.
In another bowl, whisk melted butter, egg, milk, and vanilla extract until smooth.
Pour wet ingredients into dry ingredients and fold gently until just combined.
Fold in mini chocolate chips.
Spoon batter into the mini muffin tin, filling each cup about ¾ full.
Bake for 10-12 minutes or until a toothpick inserted into the center comes out clean.
Let muffins cool for 5 minutes in the tin, then transfer to a wire rack to cool completely.