Preheat your oven to 325°F. Line 16 wells of standard muffin tins with cupcake liners or grease the wells, and set the tins aside.
In a small bowl, place the flour, xanthan gum, cornstarch, baking powder, baking soda, and salt, and whisk to combine well. Set the bowl of dry ingredients aside.
In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl with a handheld mixer, place the butter, sugar, egg, egg white and vanilla, and beat at medium-low speed to combine. Increase the mixer speed to medium-high and beat until it appears whipped (about 2 minutes).
To the large bowl with the butter mixture, add the dry ingredients in 3 batches, alternating with the milk or buttermilk, beginning and ending with the dry ingredients. Beat lightly after each addition until just combined. Do not overmix. The batter should be light and fluffy.
Fill the prepared wells of the muffin tin about 2/3 of the way full of the batter. Shake the tin back and forth to evenly distribute the batter in each of the wells.
Place the tin in the center of the preheated oven and bake until the cupcakes are very lightly golden brown and spring back when pressed lightly in the center (about 20 minutes). Don’t over bake.
Allow the cupcakes to cool for at least 5 minutes in the tin before transferring to a wire rack to cool completely. Repeat with the remaining cupcake batter. Cool the cupcakes completely before frosting and/or decorating.
For the frosting.
In a large bowl with a hand mixer or the bowl of a stand mixer fitted with the paddle attachment, place the butter. Beat on medium speed until creamy (about 30 seconds).
Add the 1 cup (115 g) of confectioners’ sugar, the salt, vanilla, and 1 teaspoonful of milk, and beat on medium speed until well-combined (about 1 minute).
Beat in more confectioners’ sugar until the consistency is thick but not stiff. Turn up the mixer speed to high and beat for about 5 minutes or until light and fluffy.
Transfer the frosting to a piping bag fitted with your favorite piping tip. Pipe the frosting on the cooled cupcakes, or use two spoons to scoop a dollop onto each cupcake.
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Notes
Nutrition information is per cupcake, excluding frosting, and is approximate and should not be relied upon.