This is my version of the thick, custard-style yogurt you usually get in the grocery store.
A half gallon of milk
1/2 to 1 cup powdered milk
1 cup of live, active culture plain yogurt
9 very clean plastic yogurt cups with lids
Something to stir with, like a fork or a spoon
Cookie sheet (optional)
IMPORTANT: Make sure that all of your equipment is very clean. Your goal is to create a fertile, inviting environment for the yogurt cultures, so it's important to make sure that there aren't other, unwanted cooties lurking around as well.
Step 1: Scald the Milk
Scald the milk by very slowly bringing it almost (but not quite!) to a boil. If you'd like, you can put the candy thermometer in now, to check the temperature as it's cooking. If you do want to use the thermometer at this stage, put it in the milk while it's still cold. If you put it in after it's started to scald, it will boil over and make a big mess.
At the scalding stage, the milk will start to look foamy and you'll see the tiny bubbles rising to the surface pretty quickly. This will happen at around 200 degrees F.
Remove it from the heat immediately. Once milk starts to boil, there's no stopping it.
Step 2: Let the Mixture Cool
Step 3: Mix in the Yogurt
Stir well to combine the yogurt and the milk, then pour the milk-yogurt mixtures back into the pan and stir it all in well.
Step 4: Cultivate Your Yogurt!
Arrange them on a cookie sheet, just to make it easier to move them around, and put them somewhere where they will maintain a temperature around 95 degrees F for an extended period.
I put mine in the oven, with the light on and the oven door closed, and it works well.
Alternately, you could put the cookie sheet on a heating pad and cover the yogurt cups with a towel.
IMPORTANT: If you use the oven, make sure everyone in the house knows not to turn the oven on. That would just be a hundred kinds of horrible.
Step 5: Go Do Something Else for 8 to 10 Hours
Let the yogurt cultivate undisturbed for about eight to ten hours. The longer you let it cultivate, the tangier and more distinctly yogurt-ey it will taste.
Test the consistency by taking a cup out and tilting it.
Once you're happy with your yogurt, refrigerate it immediately to retard the growth of the cultures.
Remember to save your last cup of yogurt for the next batch.