Cookie, in baby sign language, looks like you are using a cookie cutter to cut out a cookie from a sheet of dough. To make the sign, take your non-dominant hand and lay it out flat with palm facing up in front of your body. Bend your fingers on your dominant hand into an open circle, then place your finger tips on your non-dominant hand and twist them back and forth, as if indicating the shape of your imaginary cookie.
If your baby enjoys the occasional cookie as a treat, then teach them the sign. With highly motivating signs like cookie, children learn fast!
Cookie can be a dangerous sign to teach based on real-life cookies, because cookie consumption will dramatically escalate (for both parent and child). So instead, you can use the flash card, but beware! You and your baby will get the munchies!