Preheat the oven to 350°F (180°C). Line 2 baking sheets with parchment paper.
In a medium bowl, stir together the cookie mix, butter, and egg until well combined. Chill the dough in the refrigerator for at least 1 hour, up to 4 hours.
On a generously floured surface, use a rolling pin to roll out the dough to ¼-inch thick. Use a snowflake cookie cutter to cut out cookies, then gently transfer to the prepared baking sheet, spacing 2 inches apart. Gather the dough scraps and re-roll to cut out more cookies up to 2 more times.
Bake the cookies for 8–12 minutes, until just barely golden brown around the edges. Remove from the oven and let cool completely on the pans.
To decorate, draw a snowflake pattern on the cookies with icing, then top with festive sprinkles.