Preheat oven to 375 degrees F. Unwrap all the Reese's Peanut Butter Cups and place them in the freezer until ready to use.
In a large bowl, whisk together the brown rice flour, potato starch, tapioca starch, xanthan gum, salt, and baking soda. Set aside.
In the bowl of a stand mixer, beat the butter, sugars, and peanut butter until smooth and fluffy. Beat in the egg, vanilla, and milk. With the mixer running on low, slowly add in the dry ingredients. Mix until evenly blended.
Roll dough into 48 little balls (about 2 teaspoons per cookie) and place in a mini muffin tin. Bake in preheated oven for 8 minutes. Remove from oven and immediately press the frozen Reese's Peanut Butter cup into the middle of the cookie. Return to oven and bake for an additional 2 minutes.
Remove baking pan from the oven and sprinkle the top of the chocolate with Christmas sprinkles (optional). Let the cookies remain in the pan for 3-5 minutes* before carefully lifting them out of the pan with a sharp knife. Allow cookies to cool completely before storing in air-tight containers.
Do NOT let the cookies cool in the pan for longer than 5 minutes or they will begin to stick to the pan and not come out easily. Please learn from my mistakes. ;)
Recipe by Faithfully Gluten Free at http://www.faithfullyglutenfree.com/2012/12/gluten-free-peanut-butter-cup-cookies.html