In a medium saucepan, melt 2 cups of semi-sweet chocolate chips and 1/4 cup of unsalted butter over low heat, stirring continuously for about 5 minutes until smooth and glossy.

Remove it from the heat and quickly stir in 1 can of sweetened condensed milk and 1 teaspoon of vanilla extract until well combined.

Divide the creamy fudge base into separate bowls, depending on how many colors of Monster Fudge you want to create. Add 1-2 drops of green food coloring to one bowl, then mix until fully incorporated.

Line an 8x8-inch baking pan with parchment paper, allowing some overhang for easy removal. Spoon in the colored fudge mixtures in layers, alternating colors for a marbled effect.

While the fudge mixture is still soft, sprinkle on 1/2 cup of mini marshmallows, crushed chocolate cookies, and candy eyeballs, pressing them down lightly.

Place the baking pan in the refrigerator and chill for about 2 hours.

Once the fudge has fully set, use the parchment paper to lift it out of the pan and cut it into neat squares with a plastic knife.