Preheat oven to 350 degrees. Line two baking pans with silicone baking mats or parchment paper.
With a hand or stand mixer, beat together the butter and sugar until pale and fluffy, about 2 minutes. Add the egg and vanilla and beat until well combined.
Add the flour mixture to the butter mixture and mix on low until a smooth cookie dough is formed. Scrape down the sides of the bowl with a spatula as needed.
Chill the dough in the refrigerator for about 20 minutes.
Form the dough into balls and dip the top halves of the cookie dough balls in the red and green sanding sugar. Space the cookies evenly across the two pans. This recipe will yield between 20 and 24 cookies depending on the size of your cookie scoop.
Bake until the cookies are just barely browning on the edges, 10 - 12 minutes.
Allow to cool on pans for at least 10 minutes.