Line an 8×8-inch baking dish with parchment paper, leaving extra overhang on the sides to easily lift out the fudge once set.
In a saucepan over low heat, combine the white chocolate chips with two-thirds of the sweetened condensed milk and 1 tablespoon of butter. Stir continuously until the mixture is smooth and creamy to avoid burning and ensure even melting.
In a separate saucepan over low heat, melt the dark chocolate chips with the remaining sweetened condensed milk and 1 tablespoon of butter. Stir constantly until fully melted and smooth to maintain the right consistency.
Alternate spoonfuls of the white and dark chocolate mixtures into the prepared baking dish, creating a contrasting black-and-white pattern for the ghostly effect.
Gently swirl the two chocolates using a toothpick or butter knife to create a marbled ghostly design without overmixing, which keeps the distinct colors visible.
While the fudge is still soft, place ghost-shaped marshmallows evenly across the surface, pressing them lightly into the fudge so they adhere but retain their shape.
Sprinkle black sanding sugar over the top to add a sparkling, spooky finish that enhances the Halloween theme.
Refrigerate the fudge for at least 2 hours until it is completely firm and easy to cut without smudging the swirls.
Use the parchment paper overhang to lift the fudge from the dish, then cut into 36 even squares. Arrange on a festive platter and serve chilled for the best texture and appearance.
Variations
Swap the dark chocolate chips for milk chocolate to create a sweeter, less intense swirl.
Add a teaspoon of peppermint extract to the white chocolate mixture for a festive minty twist that complements the creamy flavor.
Use colored sanding sugar, such as orange or purple, to match different Halloween themes or party colors.
Top marshmallows with tiny dots of melted dark chocolate using a toothpick to create spooky ghost eyes and mouths for added character.
Incorporate a handful of crushed candy corn into the fudge before chilling for a crunchy surprise inside.
Tips
Store the fudge in an airtight container in the refrigerator to keep it fresh and maintain its creamy texture. It will keep well for up to 1 week.
If you prefer softer fudge, bring it to room temperature for about 10 minutes before serving.
Use parchment paper with overhang when lining the baking dish to make removing the fudge easier and cleaner.
For cleaner cuts, wipe your knife with a warm, damp cloth between slices to prevent sticking.
Keep the fudge chilled until just before serving to preserve the neat appearance of the swirls and marshmallow decorations.
When melting chocolate, use low heat and stir constantly to avoid scorching and ensure a smooth finish.
Leftover fudge can be frozen for up to 3 months; thaw in the refrigerator overnight before serving.
