2 tablespoons plain yogurt with live active cultures
In a Dutch oven, heat milk over medium heat until a thermometer reads 200°, stirring occasionally to prevent scorching. Remove from heat; let stand until a thermometer reads 112°-115°, stirring occasionally. (If desired, place pan in an ice-water bath for faster cooling.)
Whisk 1 cup warm milk into yogurt until smooth; return all to pan, stirring gently. Transfer mixture to warm, clean jars, such as 1-qt. canning jars.
Cover jars; place in oven. Turn on oven light to keep mixture warm, about 110°. Let stand, undisturbed, 6-24 hours or until yogurt is set, tilting jars gently to check. (Yogurt will become thicker and more tangy as it stands.)
Refrigerate, covered, until cold. Store in refrigerator up to 2 weeks.