Preheat the oven to 350 °F. Place paper cupcake liners in two 12 cup muffin pans.
In a large bowl beat together the butter and sugar with an electric mixer on medium speed for 1 minute or until fluffy.
Beat in the eggs yolks one at a time and add the vanilla and milk. Beat 1 minute until smooth.
In a medium bowl whisk together the flour blend, baking powder, and salt. Add in 2 batches to the egg mixture, beating just to combine each time.
In a small bowl beat the egg whites to form soft peaks. Fold into the batter.
Pour batter into prepared pans and bake until a toothpick inserted in the center comes out clean, 15 to 20 minutes. Cool in pan for 5 minutes then transfer cupcakes to a wire rack to cool completely.
Meanwhile, make frosting by beating together all ingredients until smooth and spreadable, using additional sugar as necessary for consistency. Spread on cooled cupcakes.
*Gluten-Free Flour Blend: Combine 2 cups rice flour, 2/3 cup potato starch, 1/3 cup tapioca flour and 1 teaspoon xanthan gum.
Store in container with tight-fitting lid; stir before using.
Makes 3 cups.