Set up a double boiler: fill a medium saucepan with about 1–2 inches of water and bring it to a gentle simmer over medium-low heat. Place a heatproof bowl that fits snugly over the saucepan (without touching the water) on top.

Add 4 ounces unsweetened chocolate (chopped) and ½ cup (1 stick) salted butter (cut into pieces) to the bowl. Stir or whisk constantly until the chocolate and butter are fully melted and the mixture is smooth.

Remove the bowl briefly from the heat (keep the saucepan simmering) and whisk in ½ cup half-and-half until incorporated and smooth.

Return the bowl to the double boiler. Add ½ cup of the granulated white sugar while whisking continuously; continue whisking until the sugar appears dissolved and the mixture is smooth and not grainy (about 30 seconds).

Add the next ½ cup sugar and whisk until dissolved, then add the final ½ cup sugar and whisk again until dissolved. Keep the mixture over the simmering water and stir to prevent sticking.

Cover the bowl loosely with a lid or foil and cook over the simmering water, stirring every few minutes, until the sauce becomes glossy and has thickened slightly, about 20 minutes total. Maintain a gentle simmer—do not let the water boil vigorously.

Remove the bowl from the double boiler and let the hot fudge cool slightly off the heat for about 15 minutes (it will thicken as it cools). Keep it warm for serving or transfer to a warm container.

To assemble each sundae: place scoops of ice cream of choice into a bowl or ice cream dish. Drizzle the warm hot fudge over the ice cream.

Top with whipped cream, nuts, and a cherry (or any desired toppings) and serve immediately.