In a large mixing bowl, cream butter and sugar.
Add the egg, vanilla, and salt and blend.
Add the flours and xanthan gum. Mix until well combined.
Cover and refrigerate for 1 hour.
Take the dough out of the fridge, and let it come to room temp for about 30 minutes.
Lay out some parchment paper on the counter and flour it with white rice flour. Flour your hands and use the warmth of your hands to gently knead the dough, and then roll out the dough. (Use white rice flour on the rolling pin and parchment paper to keep the dough from sticking)
Cut out your hearts and bake on a silpat (or parchment paper) lined baking sheet at 350 degrees for 11 minutes.
Let the cookies cool completely before decorating. To make the frosting, just blend the frosting ingredients until smooth. You can add more milk if you wish for a thinner, more royal icing type of frosting.