To make the hot fudge sauce, add the heavy cream, powdered erythritol, and vanilla to a saucepan. Place it over medium heat and let it warm. Remove it from the heat the moment it starts to simmer.
Step 2
Chop the dark chocolate and add it in. Stir until it melts. Taste and adjust the sweetness to your liking. Transfer to a bowl and let it sit for 10 minutes.
Step 3
To assemble, add 2 tbsp of fudge to the bottom of a sundae cup. Add 2 scoops of ice cream and drizzle 2 more tbsp of chocolate fudge. Top with whipped cream and chopped walnuts, and serve immediately!