How to Make It
Fit a medium, heavy-bottomed pot with a candy or deep-fry thermometer. Add milk to pot and place over medium heat. Cook until thermometer reads 180°F, about 10 minutes. Remove from heat and let milk cool to 115°F, about 25 minutes.
Place yogurt in a small bowl. Add ½ cup cooled milk and whisk until smooth. Add yogurt mixture to pot and whisk to combine. Cover pot with a lid and place in an unheated oven with light on until thickened, at least 6 hours and up to overnight.
Transfer yogurt to resealable jars; refrigerate, covered, for up to 1 week.
For thicker yogurt: Line a fine-mesh strainer with a double layer of cheesecloth and set over a large bowl. Pour yogurt into strainer and refrigerate until desired thickness is reached, at least 3 hours and up to overnight. Discard any liquid in bowl. Transfer strained yogurt to resealable jars; refrigerate, covered, for up to 1 week.