Soften butter or butter alternative in stand mixer. Add Sugar Cookie Mix and egg or egg replacer, mixing until dough comes together. Form dough into a log. Wrap log in plastic and chill for 30 minutes to 1 hour.
Option 1 -- Sprinkles on the outside: Put sprinkles or sugar crystals in a large, shallow dish. Roll the dough logs in the sprinkles until they are completely coated. Cut log into 1/4" slices and place slices on parchment covered cookie sheet.
Option 2 -- Sprinkles on top: Cut log into 1/4" slices and place slices on parchment covered cookie sheet. Sprinkle the tops of the cookies with sugar crystals or sprinkles.
Bake in pre-heated 350° oven for 8 to 10 minutes. When cookies are cooled, they can also be frosted.