In a medium bowl, beat ½ cup of softened unsalted butter and 4 cups of powdered sugar together until light and fluffy, around 3-5 minutes. Slowly mix in 2 tablespoons of milk or heavy cream and 1 teaspoon of vanilla extract until well combined.

Divide the frosting into separate bowls and add food coloring to each bowl, mixing thoroughly until achieving vibrant colors.

Select pairs of Chips Ahoy cookies, flip one upside down, and pipe a generous dollop of frosting onto it.

Carefully place the matching cookie on top, pressing down gently to create a sandwich.

Press candy eyes into the frosting to create monster faces, varying the number of eyes for unique designs.

Let the cookies sit at room temperature for 10-15 minutes to set slightly before storing.