Prepare your cookies by laying them out on a baking sheet.
To make your icing, combine powdered sugar with milk, corn syrup and flavor extract. If icing is too thin, add more powdered sugar, 1 tsp at a time until desired consistency is reached. If icing is to thick, add more milk 1 tsp at a time until desired consistency is reached. For best results, you want the icing to be the consistency of a very thick honey. If it's much thinner than that it will run off your cookies.
Once icing is mixed, divide it into three bowls. You will want slightly more icing in two of the bowls than you will in the third bowl, which will be used for your black icing. Leave one bowl of icing white, color one bowl yellow and the bowl with slightly less icing, black. Once colors are mixed you are ready to ice your cookies. =
Pour each of your icings into a Ziploc bag for piping. Working one color at a time, snip a very small corner off the bottom of your bag (1/4" at most). For Charlie Brown cookies, ice the flat, back side of round sugar cookies with your yellow icing. Once completed, sit aside to dry. For Snoopy cookies, ice the top of each peanut butter sandwich cookie. Set all cookies aside to dry for at least 30 minutes.
When ready to complete the cookies, knead your black icing in the Ziploc bag a bit to freshen it up. Snip a very small corner off the bottom of the bag. Gently pipe a black zig zag line on your Charlie Brown cookies, and an eye, nose, mouth and ear on to each of the Snoopy cookies. Once completed, let cookies dry for at least 6 hours prior to eating to allow icing to harden.