Prepare the Pan:
Line an 8×8 inch baking pan with parchment paper or aluminum foil, leaving some overhang on the sides for easy removal later. Lightly grease the paper or foil with butter or cooking spray.
Melt the Chocolate:
In a medium saucepan, combine the chocolate chips, sweetened condensed milk, and butter over low heat. Stir continuously until the chocolate chips are completely melted and the mixture is smooth and well combined.
Add Flavor:
Remove the saucepan from the heat and stir in the vanilla extract until incorporated.
Fold in Marshmallows:
Allow the chocolate mixture to cool slightly, then gently fold in the miniature marshmallows until they are evenly distributed throughout the mixture. If you’re adding nuts, fold them in at this stage as well.
Pour into Pan:
Pour the fudge mixture into the prepared baking pan, spreading it out evenly with a spatula or the back of a spoon. Smooth the top of the fudge as much as possible.
Chill and Set:
Place the pan of fudge in the refrigerator and let it chill for at least 2 hours, or until the fudge is firm and set.
Slice and Serve:
Once the fudge is completely set, use the parchment paper or foil overhang to lift it out of the pan. Place it on a cutting board and use a sharp knife to cut the fudge into squares of your desired size.
Enjoy:
Serve the marshmallow fudge immediately, or store it in an airtight container in the refrigerator until ready to enjoy. Bring it to room temperature before serving for the best texture and flavor.
