Preheat oven to 350°F. Line a cupcake tray with paper liners.
In a medium bowl, sift the gluten free flour, xanthan gum, and corn starch. Add baking soda, baking powder, and salt and whisk to combine.
Mix together the milk and vinegar (or lemon juice), let sit for 5 minutes.
In another bowl, whisk egg whites, egg, milk mixture, and vanilla.
Beat butter and sugar in a large bowl until light and fluffy.
Add dry ingredients and egg mixture to butter and sugar in alternating portions, starting and ending with dry.
Mix until completely combined, but do not overmix.
Divide batter amongst cupcake liners. Bake for 12 minutes or until golden brown.
Cool in cupcake tray for 5 minutes, then move to wire rack.
Once cupcakes are completely cooled, frost them with your favorite frosting and enjoy!