In a medium bowl, use an electric mixer to beat 1 cup of softened unsalted butter and 3 cups of powdered sugar together on medium speed for about 2-3 minutes until light and fluffy.

Add 1 teaspoon of vanilla extract and 2 tablespoons of milk or heavy cream to the mixture. Beat again for another 1-2 minutes until the frosting is light and spreadable.

Divide the buttercream into small bowls, adding drops of gel food coloring to each and stir until achieving desired vibrancy.

Select similar-sized Chips Ahoy cookies and pair them up, flipping one cookie from each pair upside down for frosting.

Using a spatula or piping bag, spread or pipe the colored frosting on the flat side of one cookie in each pair.

Press 2-3 candy eyes onto the sides of each assembled sandwich cookie while the frosting is still soft.

Allow cookies to set at room temperature for about 30 minutes before storing.