Step 1: Ingredients
I'm not certain if UHT milk will work (only because I haven't tried it yet) but pretty much any other cow's milk will! You can do fat free or whole milk, the one I had on hand was whole milk sourced from my local mega mart.
As for yogurt, you want to find a yogurt with live cultures. I was going to make Greek Yogurt, so that's what I picked up (didn't have any at home).
2 Large canning mason jars
Medium sauce pot
Temperature Probe / Thermometer
Sous Vide device & container
Cheese cloth / paper towels
Container to put your yogurt in
Step 2: Flame On
While you're doing so, prep your sous vide device to 109°f (42°c).
Once you hit your temperature, turn it off and take it off the stove. We now want to cool it to 109°f (42°c). You can let it air cool, but the risk is that something else in the air could infect your milk is there (if a small chance). I place the pot in to sink with a water bath with ice cubes and stir. I generally get the temperature mark in 5 - 7 minutes, depending on how many I've cubes I've got.
Step 3: Inoculation Time
Once your sous vide device is up to temperature, put your jars in and set for 5 hours. This is the minimum, and additional time will yield more tart yogurt. I've gone 7 hours with no issues, but haven't yet experimented with longer. As a TV Chef says "walk away"!
You've now got milk with live cultures that at temperature are happily eating and multiplying. Let them do their thing!
For those without a sous vide device, I've been told that you can put the jars into an oven, with the interior light on overnight. Many people do this, but I've not yet done so, so cannot offer any insight.
Step 4: Yogurt / Greek Yogurt
Now if you want to make Greek yogurt, we need to drain the excess whey. Take those jars out, and pour into cheesecloth lined colander. I elevate the colander to make sure the whey drains out. Drain for a couple of hours, then put into your container.
Either way you decide, be sure to leave a couple of teaspoons for your next batch. I hope you enjoy!