Heat oven. Preheat oven to 350 degrees Line baking sheets with parchment paper.
Combine Dry Ingredients. Whisk together the gluten-free flour, baking powder, and salt in a small bowl.
Beat Butter and Sugar. Cream together sugar, butter, and vanilla in a large mixing bowl until a thick paste forms, about 1 minute. Add egg and mix until well combined. Reduce speed to low and add the dry ingredients. Scrape down the sides and bottom of the bowl. Mix until a thick dough forms.
Chill Dough. (optional: dough is sticky and hard to roll into balls, chill for 10-15 minutes.
Roll Dough in Balls. Roll dough, about 1 tablespoon each, into a ball or use a cookie scoop.
Roll the dough into sanding sugar. Place dough onto the prepared baking sheet. Space cookies about 2 inches apart. Flatten cookies to 3/4-inch thickness with the bottom of a glass or measuring cup.
Bake. Bake cookies, one sheet at a time, until set and lightly golden brown on the edges, about 12 minutes.
Cool on a rack. Cool cookies for 5 minutes on the baking sheet. Transfer cookies to a wire rack to cool. Store cookies in an airtight container.
Gluten-Free Flour: Bob’s Red Mill 1:1 Gluten-Free Baking Flour recommended. If you use a flour blend that doesn’t contain xanthan or guar gum, the cookies might spread.
To Make Ahead: The cookie dough can be made ahead. Prepare through step four. Wrap bowl tightly with plastic wrap and refrigerate for up to two days. Let dough soften at room temperature and then roll and bake as directed.
The dough can also be shaped into balls, rolled in sugar, and flattened before baking. Freeze on the baking sheet. Once the dough is frozen, transfer to a freezer container or a large zipper-top bag.
To bake from frozen. Place frozen dough on a parchment-lined baking sheet and bake as directed. Increase baking time to about 15 minutes.