How to make a chocolate fudge/syrup sauce that does not freeze?
Alcohol Sugar Fat. Your recipe needs to be more of those and less of water; just like an ice cream.

I had luck with agave syrup as a base (maple or cornsyrup?) that I tested by freezing; when I was satisfied by my choice, I melted in chocolate and additional fat (good coconut oil I think).

Certainly won't be fudgy outside of freezer so finding a recipe is tricky...

Have a look also at that ice cream topping chocolate hard-shell. It has a super low melting point with its high fat content. Wouldn't take much more sugar in the form of a syrup to make it fudgy.