Bring 2 cups+ milk to a boil in a medium saucepan, stirring constantly to prevent scorching. Let milk foam up to the top of the pan; remove from stove; lower heat to medium; return milk to the burner and boil for 1 minute, stirring constantly. Now cool milk to 100F. Use a candy thermometer and don't let milk cool below 100F. Quickly pour milk into prewarmed Thermos. Gently stir in the unflavored yogurt. Cap the bottle and leave it undisturbed for 3 to 5 hours. For a thicker yogurt add 3 to 5 more tsp. of milk powder to the liquid pint before heating. Save 2 tsp. of the batch for next time.