What It Is
Hot fudge is basically chocolate that has been turned into a syrup through the addition of sugar, and sometimes milk or cream. Chocolate can certainly have a place as part of a Paleo lifestyle, as can cream on occasion, and maybe even milk. Sugar is a little more difficult to justify, especially when it comes in the form of high-fructose corn syrup (as it usually does in hot fudge).
The Problems With Sugar
When we say chocolate is Paleo, we usually mean dark chocolate: the stuff that’s more cocoa than sugar. By taking something that already has sugar and adding more sugar, we’re asking for a major sugar rush. This can leave a lot of people feeling lightheaded and even inflamed (sugar is inflammatory), not to mention all the negative feelings associated with the crash that comes after the sugar rush (fatigue, hunger, short temper, etc.)
Mark Sisson has even more reasons to keep your sugar intake to a minimum.