In a medium bowl, combine 1 cup of softened unsalted butter with 4 cups of powdered sugar. Beat with an electric mixer on medium speed for about 2–3 minutes until light and fluffy. Slowly add 1-2 tablespoons of milk and 1 teaspoon of vanilla extract, mixing until smooth and spreadable.

Divide the frosting into small bowls based on desired colors. Add drops of food coloring to each bowl and mix until colors are vibrant. Set aside.

Match pairs of Chips Ahoy cookies by size. Flip one cookie in each pair upside down. Use a spoon or piping bag to dollop 1-2 tablespoons of frosting on the upside-down cookie and press another cookie on top to create a sandwich.

Press candy eyes onto the frosting edges of each cookie sandwich while the frosting is still soft for a whimsical look.

Allow cookies to sit at room temperature for about 10–15 minutes to let the frosting set. Serve immediately or store in an airtight container.