Preheat the oven to 350°F. Grease or butter a mini muffin pan and set aside.
In a large mixing bowl, whisk together the melted butter, sugar, egg, milk, and vanilla extract until smooth.
Add the flour, baking powder, and salt. Stir just until combined, then gently fold in the mini chocolate chips or sprinkles. Be careful not to overmix.
Fill each mini muffin cup about halfway full.
Bake for 11–13 minutes, or until the tops spring back when lightly pressed or a toothpick inserted into the center comes out with a few moist crumbs.
Let the muffins cool in the pan for 2–3 minutes before transferring them to a wire rack.
Store cooled muffins in an airtight container at room temperature for up to 3 days.