This is real soda-fountain hot-fudge sauce. It will harden on the ice cream to a thick, chewy consistency. Use Dutch-process cocoa only.
Cook the cream, butter, and sugars in a heavy-bottomed pot over medium heat until sugars are dissolved, about 5 minutes.
Lower the heat. Whisk in salt and cocoa powder and keep whisking until smooth. If any lumps remain, strain them out. Serve immediately, or refrigerate and then microwave to reheat.