Line a 9 x 13 inch pan with foil and then grease lightly with butter. Side Note: You can also use a 9 x 9 pan if you would prefer thicker squares over thinner bars.
Place chocolate chips, sweetened condensed milk, and vanilla extract in a bowl then microwave for 1 minute. Stir and microwave in 15 second increments until chocolate is melted and smooth. Don't overcook!
Add two cups of the marshmallows and most of the nuts to the chocolate mixture and stir until well combined.
Pour the mixture into the prepared pan and smooth until flat.
Sprinkle the remaining marshmallows and nuts to the top of the fudge, pressing them in lightly.
Place into the refrigerator for 2 hours or until set.
Cut into squares or bars. Store in an air tight container in the fridge. Enjoy.