Prepare an 8x8-inch baking pan by lining it with parchment paper, leaving an overhang to pull the fudge out of the pan. Very lightly spray the parchment paper with nonstick cooking spray.

Combine the sugar, butter, heavy cream, and corn syrup in a heavy-bottomed saucepan. Cook over medium heat, stirring constantly, until the mixture reaches a rolling boil.

Once boiling, continue stirring and allow it to boil for about 4-5 minutes, or until a candy thermometer registers around 235-240°F (soft-ball stage).

Remove the saucepan from heat and stir in the chopped white chocolate until it's completely melted and smooth.

Add the marshmallow cream and vanilla extract to the mixture, stirring until well combined.

Pour ¾ of the fudge mixture into the prepared baking dish.

Add the unsweetened cocoa powder to the remaining fudge. Whisk until fully incorporated.

Carefully spoon the cocoa-flavored fudge mixture on top of the white layer.

Use a toothpick or skewer to swirl the two layers, creating a marbled effect.

Let the fudge cool at room temperature for about 1 hour. Cover with aluminum foil. Then, chill in the refrigerator for 1 hour or until firmly set. (If your kitchen is on the colder side, you can allow the fudge to finish setting on the counter)

Once set, lift the fudge from the pan using the parchment paper overhang. Use a sharp kitchen knife to slice the fudge 5 x 5 slices. (You can also cut the fudge 6 x 6 slices)

Add the chocolate chips and vegetable oil to a microwave-safe bowl. Heat on high for 30 seconds, stir, and heat in 15-second intervals until the chocolate is completely melted.

Dip the tops of the fudge in the melted chocolate. Allow the chocolate to completely set before serving.