In a large bowl, blend the butter, sugar, egg, egg white and almond extract until well combined. Add the flour, almond flour, xanthan gum, baking powder, and salt, beating well after each addition. After adding the final ingredient, beat the mixture until it becomes thicker and a bit more elastic. Divide the dough into two equal parts, wrap each part in plastic wrap, and place the dough in the refrigerator to chilll for an hour until firm.