Preheat oven to 350 degrees F. Place cupcake papers in a cupcake pan.
Sift the flour, cornstarch, baking powder, baking soda and salt into a bowl, and whisk together.
Whisk the soften butter and sugar until light and fluffy. Add the egg and vanilla extract and mix again.
Add all the dry ingredients and give it a stir. Add the milk and mix until combined well.
Fill the cupcake papers about 3/4 of the way up. Bake for 20-23 minutes at 350F degrees or until golden-brown color on top.Check the readiness of the cupcakes with a skewer or a toothpick and transfer the cupcakes onto a cooling rack and let them cool completely.