Heat the milk in a medium (2-quart) saucepan over medium heat, stirring often with a wooden spoon. Monitor the temperature of the milk with an instant-read thermometer until it reaches the desired temperature. For pourable, European-style yogurt, heat to 165ºF/74ºC. For custard style yogurt, heat to 195ºF/90ºC.
Hold the milk at the desired temperature for 10 minutes. In order to do this, immediately turn the burner down to the lowest heat setting, and stir often, checking the temperature frequently and raising/lowering the heat as needed. When the ten minutes are up, remove the saucepan from the heat.
Place a 10-inch baking pan in the sink, and fill with cold tap water. If you want to reduce the cooling time considerably, add two cups of ice cubes to the water. Gently place the saucepan into the baking pan full of tap water, taking care not to splash any water into the milk. Let cool to 115ºF/46ºC, stirring occasionally to hasten the cooling process. Without ice cubes, this will take about 20 minutes; with ice cubes, the milk will cool much more quickly, in about five minutes. While the milk is cooling, set the folding bread proofer to 120ºF/49ºC.
Pour 1 cup of the milk into a 2-quart glass measuring bowl, and add the live, active yogurt. Whisk gently and slowly to combine the yogurt and milk thoroughly, without creating excess froth. Add the remaining milk to the measuring bowl and whisk gently until thoroughly mixed.
Pour the milk/yogurt mixture into two pint-sized Mason jars. Place in the folding bread proofer. After one hour, lower the temperature to 86ºF/30ºC. After two more hours, begin to check the yogurt for doneness. You can do this by tilting a jar of yogurt to see if it has set up. If you do not have a folding bread proofer, you can use an insulated cooler, heated with warm water and kept at temperature with hot water bottles.
Screw the lids onto the jars of yogurt and place in the refrigerator, chilling completely before serving. Store for up to two weeks.