Preheat oven to 350 degrees Fahrenheit. Line a muffin tin with 12 paper liners. Set aside.
In a large bowl, combine gluten-free flour blend, xanthan gum (if using), cornstarch, baking powder, and salt. Whisk to combine. Set aside.
Cream butter and sugar in a bowl with an electric mixer until light and fluffy, about 2 minutes.
Add eggs and continue mixing until eggs are incorporated. Then, add milk and vanilla extract and mix until combined.
Gradually add in the flour mixture and mix until no streaks of flour remain.
Pour cupcake batter into the cups with either a spoon or an ice cream scoop, about 2/3 of the way full.
Bake for 20-25 minutes, or until golden brown. Cool completely.