Bake sugar cookies (if you're making homemade sugar cookies). Allow to cool completely.
Using an offset spatula, spread a thin layer of frosting over the top of each cookie. A thin layer is easier to spread smooth with no rim around the edge than a thicker layer.
Evenly divide the remaining frosting between the number of colors you want to use for the irises, with about 1/4 cup leftover for the black pupils and white reflections.
Tint the frosting for the irises. Spread a thin layer over the center of the cookies.
Place a few dollops of food coloring (the same as you used for the irises) onto a plate.
Using a small, food-safe brush, paint the food coloring onto the colored part of each eye working from the outer edge inward.
Color half of the remaining frosting black. Pipe a small dollop onto the center of each eye to create the pupil.
Swirl the black frosting into place using a toothpick or gently spread with an offset spatula.
Pipe two small dots of white frosting onto the pupils, using a toothpick to pull the bottom dot down and in.
Notes
If you're baking your cookies, make sure they've cooled completely before you begin frosting.
Sugar cookies lost their shape while baking: Re-cut the cookies with the same cookie cutter immediately after removing the tray from the oven.
Smooth frosting: The easiest way to get a nice, smooth layer of frosting is to place the cookies on a revolving cake stand and use a small offset spatula to smooth the frosting as you turn the top.