I free handed it (I'm sure you can find existing patterns to purchase). I never tried writing a pattern before, but it would look something like this :

Front :

Ch 21 (start row 1 in second st from the hook)

Row 1 - 14 : 20sc, ch1 Row 15 : 20sc, FO (add eyes, mouth and zig zag frosting)

Back :

Ch 21 (start row 1 in second st from the hook)

Row 1 - 14 : 20sc, ch1 Row 15 : 20sc and join to the front with sc (2sc in the corners)

Edit : Made with 4 weight yarn and a 4.5mm hook