Preheat the oven to 350°F and line a 12-cup muffin tin with cupcake liners.

In a medium bowl, sift together the gluten-free flour blend, baking powder, salt, and xanthan gum.

In a large bowl, beat the softened butter and sugar with an electric mixer for 3 to 4 minutes until light and fluffy.

Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.

In a small bowl, whisk together the milk and apple cider vinegar and let sit for 1 minute to curdle.

Add half of the dry ingredients to the butter mixture and stir until just combined, then add half of the milk mixture and stir gently. Repeat with remaining dry and wet ingredients, mixing until no streaks remain.

Divide the batter evenly among the prepared liners, filling each about two-thirds full.

Bake at 350°F for 16 to 18 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Cool the cupcakes in the tin for 10 minutes, then transfer to a wire rack to cool completely before frosting.