Preheat the oven to 210℃/425℉. Grease or line a cupcake mould.
Add the melted butter, milk, eggs, and vanilla extract into a bowl. Whisk till combined. Set aside.
Sift in all-purpose flour and baking powder into another bowl.
Add sugar and chocolate chips to the flour. Combine with a spatula.
Add the wet ingredients to the dry ingredients in 2-3 parts while mixing with spatula in between.
Spoon the batter into lined cupcake mould, filling them to the brink. Sprinkle chocolate chips and sugar on top.
Bake in the preheated oven at 210℃/425℉ for 10 minutes then reduce the temperature to 180℃/350℉ and bake for 15-20 minutes or until the top becomes golden and a skewer inserted into the muffin comes out clean.
Transfer the muffins to a wire rack to cool.