Decorate cookies. For eyes, use tip 12 and full-strength white icing to outline and fill in eyeballs. Pat smooth with finger dipped in cornstarch. Use tip 5 and full-strength black icing to pipe pupils. Pat smooth with finger dipped in cornstarch.
For mouth, use tip 5 and full-strength black icing to outline and fill in. Pat smooth with finger dipped in cornstarch. Use tip 3 and full-strength black icing to add smile lines and creases. Let set 1 to 2 hours.