"Making yogurt in a slow cooker is foolproof and since it works just as well as a yogurt maker, it takes away the need for yet another gadget in your house! Yogurt usually keeps in the fridge for up to 2 weeks."
Place four 10-ounce jars in a slow cooker. Fill slow cooker with water up to 1/3 inch of the jar rims. Remove jars and set aside.
Cover slow cooker and preheat on High while you prepare the yogurt for about 20 minutes.
Pour milk and milk powder into a saucepan and whisk to dissolve. Heat over medium heat until just steaming and a thermometer reads 179 degrees F (82 degrees C), about 10 minutes. Remove from heat and place the bottom of the pan into a bowl of ice water. Cool milk mixture until the temperature has dropped to 114 degrees F (46 degrees C). Lift pan out of the ice water.
Measure about 1 cup of milk and pour into a bowl. Stir in yogurt until thoroughly blended. Return yogurt mixture to the pan of milk. Pour milk mixture into the 4 glass jars, filling up to 1/2 inch of the rims.
Check the temperature of the water in the slow cooker is no higher than 114 degrees F (46 degrees C). If it is cooler, cover slow cooker again, and allow to come to temperature. If it is too hot, uncover and turn off slow cooker.
Set jars in the warm water; the water level should be up to the level of the yogurt in the jars. Cover with a folded dish towel and then with the slow cooker lid. Turn slow cooker off. Allow to set for 6 hours, keeping the water at 114 degrees F (46 degrees C) as best you can. Check the temperature with your thermometer after about 2 hours, taking care not to disturb the yogurt. If the water is lower than 114 degrees F (46 degrees C), turn slow cooker to Warm for 10 minutes, then shut off again. Check temperature again at the 4-hour mark and repeat. Do not stir or poke the yogurt at all during this time.
Check the yogurt after 6 hours by pressing gently on the top or tipping the jars to see if it is set; it is done when the yogurt is firm and there is a thin layer of yellowish liquid on the top. Remove the jars from the water and dry off. Top with clean lids and place in the refrigerator.
Be sure to save a little yogurt as a starter for the next batch. Over time your homemade yogurt will start to lose its potency as starter, so every fourth or fifth batch you may need to use store-bought yogurt as starter.
You can skip the milk powder, but your yogurt won't be as thick. Likewise, you can increase or decrease the amount of milk powder used to achieve thicker or thinner results.
For a super-thick, Greek-style yogurt, simply strain your yogurt after it has set. You can place muslin or a coffee filter in a fine sieve, then add the yogurt and set it over a bowl to catch the liquid. Strain overnight in the fridge.