In a mixer bowl using an electric mixer or stand mixer, combine powdered sugar and 4 teaspoons of milk, starting the mixer very slowly on low. As mixture starts to combine, add in corn syrup. Beat well to combine. If mixture is too thick, add more milk, about 1/2 tsp at a time (a tiny bit can make a difference so go slow!). Beat until frosting is smooth and creamy.
Divide frosting into multiple bowls if desired, and add color and flavor to each bowl. For white frosting, it's best to stick with a clear extract such as peppermint or lemon. For the entire recipe, start with about 1/2 tsp of your extract of choice and flavor to taste. With food coloring, start with one to two drops, and increase until desired shade is reached.
Frosting can be adjusted to meet your desired consistency. If you want a more liquid like "flood" icing, add more milk, a tiny bit at a time. If you want a thicker frosting to spread on, slowly add more sifted powdered sugar, approximately a teaspoon at a time. This frosting is very flexible and forgiving. If frosting thickens as you are working with it, you can always add a few more drops of milk and stir well to combine. Allow cookies 4-6 hours for frosting to dry so that it is firm to the touch.