Cold. And it's important that you follow this rule if you want good results. The only thing you do after you take it out of the refrigerator is to soften the caramel in the bottom; otherwise, you won't be able to remove it completely. For this, you put the pan over a low flame or burner until the flan's bottom releases. You will feel it if you gently shake the pan. There is a step-by-step video showing all of this in the post.