How-to for Marble Icing on Cookies
Dip in Royal Icing
For marbled cookies, you need Royal Icing. Royal Icing is a fairly runny icing that dries smooth and hard. For swirls, divide your Royal Icing in two bowls. Tint one, and leave the other white (or tint it a contrasting color). Frost cookies with a solid-color base. The first technique is dipping the cookie facedown into the bowl of icing. Let the excess drip off, then quickly flip the cookie over and set it on a rack.