Combine 3 tablespoons of the milk along with the other icing ingredients (except food coloring and sprinkles) into a medium bowl. Whisk until smooth, adding more of the milk as needed.
Frosting should drizzle from a smooth ribbon from the whisk and hold its shape on top of the bowl of icing before the drizzles start to soak back into the icing.
Adjust consistency: If it’s too thick, add a little more milk (1 teaspoon at a time), if it’s too thin, add a little more powdered sugar (a few tablespoons at a time). Note that thicker icing will get used up faster, so that may affect yield if it is too thick.
Optional: If you want to decorate the cookies as shown, set aside about ¼ cup frosting and leave it white.
Color the remaining icing green with a small amount of green food coloring.
Hold a cookie upside down by the edges and dip face-first into the icing. You are just dipping the front of the cookie.
Lift and allow the excess icing to drip for 3-4 seconds. Quickly flip the cookie, gently wiggle it to spread icing, and set on a wire rack or lined cookie sheet to set. Pop any bubbles or swipe away drips quickly with a toothpick.
Decorating ideas: If you want sprinkles, add them immediately. The icing begins hardening quickly.If you want to pipe designs on the cookies, then wait for the green layer to dry completely (a few hours). Then, fill a small piping bag fit with a small round tip (or just cut the very end of the piping bag) and pipe designs onto your cookies with the white icing. Allow cookies to harden at room temperature for several hours (4+ hours ideally) before carefully stacking and storing. I like to place a piece of wax paper between each layer when storing. I’ve carefully stacked multiple layers of cookies with no problems.