You could make a ton of this hot fudge to give as a holiday gift. Or...you could slather it on vanilla ice cream, pile on the whipped cream and pistachios, and top it with a cherry (or two). Either way, happiness ensues.
Whisk the cream, sugar, and corn syrup in a large, heavy-bottomed pot set over medium heat until dissolved, 1 minute. Whisk in the chocolate and salts and bring to a boil.
Reduce heat and simmer, stirring occasionally to avoid scorching the bottom, 40–45 minutes. Toward the end of the cooking time, the chocolate mixture will begin to separate, with the oil on top of the solids. This is, in fact, what you’re looking for. Turn off the heat.
With the heat off, add the butter and vanilla to the separated chocolate mixture. Whisk to bring the fudge together, 1–2 minutes. It'll be smooth, silky, and homogeneous. If it's very thick, you can purée it in a food processor, adding water 1 tablespoon at a time, until it's smooth. Makes 4 cups. (Can be stored in the refrigerator up to 6 months. For gift-giving, spoon cooled fudge into Mason jars with tight-fitting lids, and include a reminder to refrigerate.)
Sundae assembly: Pour 1 tablespoon cherry juice into the bottom of a sundae glass. Add 2–3 scoops of ice cream, followed by 2–3 tablespoons (or more!) of hot fudge. Stir some cherry juice into the whipped cream and spoon it on top. Finish with pistachios, a drizzle of cherry juice, and two cherries. Serve immediately.