Preheat the oven to 350F degrees (180C). If your oven is a convection oven, be sure that the temperature is reduced to 325F (160C) degrees (some convection ovens to this automatically).
Line the 6 middle cavities of a regular-sized muffin tin with muffin papers.
In a medium bowl using a hand-held electric mixer, beat the egg and sugar for about 5 minutes or until the mixture is very pale in color and has significantly increased in volume.
Sprinkle the flour, baking powder and salt over the eggs and gently whisk in the dry ingredients.
Add the butter and milk to a liquid measuring cup. Microwave in 30-second increments until the butter is melted. Whisk in the vanilla extract.
Whisk the warm milk mixture into the batter until smooth.
Spoon the batter into the cupcake liners filling each about 2/3 to 3/4 full. You should end up with 6 cupcakes.
Bake in the preheated oven for 18-20 minutes or until the tops look set and are slightly firm/springy to the touch, or an inserted toothpick comes out clean.
Cool the cupcakes for 10 minutes in the pan, then remove from the pan and continue cooling on a cooling rack.