I made the Thick Caramel Filling here on cakecentral. Turned out great. Let it cool for an hour and now there's a layer of oil on top because the butter separated. What do I do? Do I need to throw it all out, or can I use it?

Does it stir back in?

Not really. I reheated it over low heat and tried to stir it back in, but it didn't incorporate. I'll try wiping off the extra oil and seeing if it still separates further.

Wiping off the "extra" oil can cause the caramel to become hard. It needs the fat to remain a sauce rather than a brittle or toffee. I don't know if this will work once the caramel is already cool, but if I see separation while it's still cooking, I'll add 1-2 tbsp of hot cream to help it come back together....although that doesn't always work. Still...you might try rewarming the sauce on med-low and add hot cream (start with 1 tbsp) and see if it helps. At this point, it can't hurt, right?