Pour 1 gallon of milk in a 5 quart slow cooker. Place on low for 2-3 hours or until it is 180 degrees. Do not let the milk boil! You can use any kind of milk–skim to whole. I prefer skim for the lower calories and we are used to it. Whole milk yogurt is delightfully creamy though.
Turn off the slow cooker and let the milk sit until it has cooled to 110-120 degrees. (2-3 hours) (You have just killed any bacteria that was in the milk previously so it won’t interfere with the new bacteria cultures that you are going to introduce.) This also unravels the proteins and allows the yogurt to thicken.
Take 1/2 cup of plain yogurt (reserved from your last batch or purchased) and mix it with 1 cup of the warm milk from your slow cooker. Then stir this mixture into the rest of the warm milk. Cover with the lid and wrap in bath towels to insulate. Keep the slow cooker turned off and allow it to sit overnight. In the morning it will look like this:
You can see the whey separated and floating on the top with the yogurt solids underneath. If you stir all this together, you will have regular plain yogurt.
To make Greek Yogurt: Layer a large bowl with a large colander and line with 3 layers of cheese cloth. Pour your yogurt into this and allow to drain without stirring until half of the volume is reduced. Save the nutritious clear whey for baking and use it like buttermilk in pancakes, biscuits, bread etc.
Take the strained yogurt and put it in your electric mixture with the wisk attachment and whip. Add a little fresh milk (or cream) until it has the moistness and consistency you like.
It should be very thick and creamy when you are done. This recipe yields 1 gallon of plain yogurt or 1/2 gallon of Greek Yogurt for around $3. This is roughly 1/4 the cost of buying the same amounts and requires very little hands on time.