Preheat oven to 350°. Line 24 muffin cups with paper liners.
In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. In another bowl, whisk flour, baking powder, xanthan gum and salt; add to creamed mixture alternately with milk, beating well after each addition.
Fill prepared cups three-fourths full. Bake 15-20 minutes or until a toothpick inserted in center comes out clean. Cool in pans 10 minutes before removing to wire racks to cool completely.
For frosting, in a large bowl, beat butter, cream cheese, confectioners’ sugar and vanilla until smooth. Gradually beat in enough milk to achieve desired consistency. Frost cupcakes. Refrigerate leftovers.