I've been making yogurt for some time now and had this idea. I finally tried it out. Unfortunately, my yogurt making is less precise than my beer making. I eye ball the entire process and always end up with good yogurt. So here's what I did.
I heated up 1 quart of 2% milk to 180F with a handfull of bittersweat chocolate chips. I would estimate it was 3/4 of a cup of chocolate chips. As it approached 180F, I dropped in one cinnamon stick (2-3" long or so). I covered and killed the heat. We then ran some errands and were gone over an hour. When we got home it was still a little warm, so I finished cooling it to 100F and pitched 2 tablespoons of a previous yogurt together with a teaspoon, or so, of vanilla extract. I whisked it all together and put it into a 1qt mason jar. It then went into the oven with only the oven light on and stayed there overnight. I put it in the fridge first thing in the morning.
My 4 year old loves it. I'm still back and forth on it. It's sweet, it's bitter, and it's spicy like mexican hot chocolate, but then it's tart, like yogurt. I enjoy it mixed with berries or bananas, but on it's own, it's a bit strange to me.
It's much creamier than my other yogurts though. That's probably due to the fat in the chocolate.
Just thought I would share in case there are any Mexican hot chocolate lovers out there who like making yogurt.