Is there any way to thicken chocolate ganache that didn't set?

Reheat in a Baine Marie and add more chocolate.

Oh! And add some more butter in too, as you're way down on fat content... You might get a nice sauce out of it... Maybe best to start again?

That'll make a difference! If it's not cream, it's not caramel in my book! How runny is your mix? If its very runny and nowhere near anything that looks like it might set, pour it all into a wide pan (to help evaporation) put it on a low heat and stir stir stir...it might not work, and you won't have nearly as much topping, but i think it's your best bet.

10 1/2 ounces milk chocolate, chopped (2 cups)
1 1/2 cups granulated sugar
1/4 cup water
3 ounces (6 tablespoons) unsalted butter
1 cup heavy cream
1/2 teaspoon table salt
1 tablespoon sea salt, preferably fleur de sel

Here's the recipe, but I used 2% milk because I had no cream. Is that the problem?

Looks like reviewers of the recipe have had similar issues....probably because of not boiling the water off the sugar? Anyway, I figure you've got two options! Either be brave, and cook the ganache till it goes thick (I'll be honest, not sure how this will go with the chocolate in it, you could ry with a very small amount) or secondly (and if it were me, my preferred option) would be to add more chocolate, add melted into room temp ganache and give it a good stir- it'll firm things up when the chocolate comes down in temp. It sounds like others recommend putting it in the fridge to get it looking vaguely like the ars in the pic!

I don't see a chocolate ganache component in the recipe -- do you mean the caramel didn't set?