In a medium bowl, using an electric mixer, beat 1 cup of softened unsalted butter and 4 cups of powdered sugar together on medium speed until light and fluffy, about 2-3 minutes. Add 2 teaspoons of vanilla and 2-3 tablespoons of milk or heavy cream to achieve creamy consistency.

Divide the frosting into separate bowls based on colors desired. Add food coloring to each bowl, stirring well until colors are bright and vivid.

Take 36-40 Chips Ahoy cookies, match them into pairs based on size, and flip one cookie of each pair upside down.

Using a piping bag or knife, pipe or spread a dollop of colorful frosting onto the flat side of the upside-down cookie. Press the matching cookie on top to create a sandwich.

While the frosting is still soft, press candy eyes into the sides of the frosting on each assembled cookie.