Delicate recipe. I would not recommend letting it cool but rather just mixing in the chocolate just after taking off the heat. By letting it cool, you'll get a fudge that is going to instantly turn before you can get it into the pan. Marbling is not an effect because the caramel is so dark already. If it does fail and turns to sand, then just put the chunks into a microwave safe bowl and nuke for 2 minutes until it's bubbling again. Then pour hot mix into pan right away and you have an instant recovery!