Prepare. Gather your ingredients as indicated. Line your baking pan (or dish) with parchment paper that overhangs the sides of the pan. Then give the parchment paper a quick spray with nonstick.
Heat. In a large microwave-safe bowl, heat together the chocolate, chips butter, and sweetened condensed milk, stopping to stir every 30 seconds, until the ingredients are melted and smooth. Note that this step can also be done using the double boiler method
Hot Chocolate Mix Carefully remove the ingredients from the microwave and stir in the hot chocolate mix, one packet at a time, until combined. Then stir in the vanilla extract.
Marshmallows. Use a rubber spatula or similar item to fold in three cups of the marshmallows.
Transfer. Transfer the combined ingredients to your prepared pan from the first step and evenly spread the ingredients out using the backside of a spoon or your spatula. Then gently press in the remaining marshmallows on top of the fudge.
Set. Place the fudge in your refrigerator to fully set (about two to three hours). Then use the extended parchment paper to lift the fudge out of the pan and cut into squares.