Step 1: Prepare the Pan

1. Line an 8×8-inch baking pan with parchment paper, leaving enough overhang on the sides to easily lift the fudge out once it’s set.
2. Lightly grease the parchment paper with nonstick spray or a small amount of butter to prevent sticking.

Step 2: Melt the Chocolate

1. In a medium saucepan, combine the chocolate chips, sweetened condensed milk, and butter.
2. Place the saucepan over low heat and stir continuously with a silicone spatula until the chocolate is completely melted and smooth. Be careful not to overheat, as chocolate can burn easily.

Step 3: Add Vanilla Extract

1. Once the chocolate mixture is fully melted and smooth, remove it from the heat.
2. Stir in the vanilla extract, ensuring it is evenly incorporated into the mixture.

Step 4: Pour into the Pan

1. Pour the melted chocolate mixture into the prepared pan, spreading it out evenly with a spatula.
2. Tap the pan gently on the counter to remove any air bubbles and level the surface.

Step 5: Add the Marshmallows

1. Sprinkle the mini marshmallows evenly over the surface of the fudge. Gently press them into the chocolate mixture so they adhere as the fudge sets.
2. If desired, add additional toppings like crushed candy canes, chocolate shavings, or festive sprinkles.

Step 6: Chill and Set

1. Place the pan in the refrigerator and let the fudge chill for 3–4 hours, or until it is completely firm.
2. Once set, use the parchment paper overhang to lift the fudge out of the pan.

Step 7: Slice and Serve

1. Using a sharp knife, cut the fudge into small squares. For cleaner cuts, wipe the knife with a damp cloth between slices.
2. Serve immediately or store in an airtight container for later.