In a medium mixing bowl, combine 1 cup of softened unsalted butter and 4 cups of powdered sugar. Use an electric mixer on medium speed for about 2-3 minutes.

Mix in 2 teaspoons of vanilla extract and 2-3 tablespoons of milk or heavy cream. Beat until light and fluffy.

Divide the frosting into separate bowls, add food coloring, and mix until achieving vibrant shades.

Pair Chips Ahoy cookies by size, setting half upside down for frosting application.

Using a piping bag or spatula, spread a generous amount of colored frosting on one cookie and top with another.

Gently press the cookies together, allowing some frosting to peek out.

Press candy eyes into the frosting while soft to create monster faces.

Let assembled cookies sit at room temperature for about 30 minutes to set.

Store leftovers in an airtight container.