Lightly oil an 8-in (20 cm) square baking pan, then line with parchment paper.
Bring a large pot of water to a boil, then prepare the fondues according to the packaging instructions.
Remove the cans from the boiling water and pat dry. Carefully open the Original Dark Fondue and pour the chocolate into a large mixing bowl. Add 1/2 cup (125 ml) of the heavy cream, the melted butter, and the salt and whisk to thoroughly combine.
Open the Salted Caramel Fondue can and pour it into a small bowl. Add the remaining heavy cream (1/4 cup / 60 ml) and whisk to combine.
Pour half the dark chocolate mixture into the prepared pan and use a spatula to spread it all over the bottom of the pan. In a zigzag pattern, pour about half of the caramel mixture over the chocolate layer. Cover with the remaining chocolate mixture, then finish by drizzling the remaining caramel mixture on top. Pull the tip of a knife through the layers to create swirling patterns.
Let cool for 10 minutes, then lightly sprinkle with sea salt flakes, if using. Refrigerate for at least 3 hours, or until the swirl fudge is fully set.
Unmold the swirl fudge. Warm up the blade of a sharp knife in hot water, then use it to cut the fudge into small squares. Rinse and wipe the blade of the knife between each cut to create clean pieces of fudge.