Preheat oven to 300°F. Line a baking sheet with parchment paper. In a small bowl, whisk together flours, cornstarch, salt, baking powder, and xanthan gum.
In the bowl of a stand mixer fitted with the paddle attachment, cream butter, sugar, and vanilla together on medium-low speed, until a thick paste forms, about 30 seconds. Shut off the mixer.
Add dry ingredients to the butter mixture and mix on low until dough forms, about 3 minutes. Divide dough in half, wrap in plastic, and chill for about 20 minutes.
Lightly flour a work surface with rice flour. Place one of the dough halves onto the surface and roll to ¼-inch thick. Using a 2½-3 inch square cookie cutter, cut out dough. Re-roll scraps as necessary, chilling before rolling, if needed. Place cookies 1 inch apart on the baking sheet. Chill cookies on the baking sheet for 20-30 minutes.
Bake for 24-28 minutes or until aromatic, edges are light brown, and centers are set. Cool on the baking sheet for 5 minutes. Transfer to a wire rack to cool completely. Repeat using remaining dough.
To Decorate: Make a batch of Royal Icing (recipe follows) and divide into three or four portions. If desired, leave one portion white. Set aside. Tint remaining portions with food coloring, as desired.
Divide each color of icing in half. To half of each color, add water, 1 teaspoon at a time, until icing reaches a drizzling consistency. Use a thin off-set spatula to spread thinned icings on cookies. Place cookies on a wire rack set over waxed paper. Let stand for 1-2 hours or until dry.
Place the other half of each color of the remaining tinted icings into separate resealable small plastic bags. Cut a small hole in one corner of each bag. Pipe lines and bows on dry iced cookies. Let stand about 1 hour until dry and ready to package as gifts.