To make the cookies: 

1. Using a stand mixer with a paddle attachment (or a hand mixer), cream together the butter, egg, vanilla, and sugar. 
2. Add the flour blend, baking powder, and salt. Mix slowly until a dough ball forms, scraping down sides of bowl as needed.
3. Cover the dough with a towel or plastic wrap and chill in the refrigerator for 1-2 hours.
4. Preheat the oven to 350 degrees. Line two baking sheets with parchment paper.
5. Sprinkle your work space and rolling pin with additional gluten-free flour (this prevents the dough from sticking). Place the chilled ball of dough on the floured surface, and use your hands to press the dough down into a disc shape. Sprinkle dough with a little more gluten-free flour. Using a rolling pin, roll dough out to ¼-inch thickness.
6. Use cookie cutters to cut the dough out into shapes. Carefully slide a metal spatula under the cookie shapes, lift up, and transfer onto the prepared baking sheets, at least 1 Inch apart. Re-roll the dough into a ball, then flatten into a disc, sprinkle with flour, roll out, and cut out more cookie shapes. Continue this process until all dough is used up.
7. Bake one or two baking sheets of cookies at a time until the cookies are light golden on the bottoms, 8 to 10 minutes. (Cooking time varies, so start checking at 8 minutes; may need to go as long as 12 minutes, depending on the thickness of the cookies).
8. Let cookies cool directly on the baking sheets for 5 minutes before removing cookies to wire racks to cool completely.

To make the piping frosting:

1. In a medium bowl, combine the powdered sugar, softened butter, heavy cream, and vanilla.
2. Using a stand or hand mixer on the lowest setting, gently begin to whip the frosting until creamy. (TIP: If the frosting is too thin for piping, add more powdered sugar (2 tablespoons at a time) until the right consistency is reached. If the frosting is too thick and difficult to pipe from the bag, add more cream (1 teaspoon at a time) until the right consistency is reached.)
3. Transfer the frosting to a frosting bag with a tip (or a plastic bag, then cut the corner tip off), and pipe frosting onto each cookie. If desired, sprinkle coarse/sanding sugar over the piping. Gently tip each cookie to remove excess sugar. Store cookies in a covered container at room temperature for up to 5 days.