Heat milk in a medium saucepan over medium heat, stirring and scraping bottom of pan often with a heatproof spatula (for a smooth yogurt, take care not to incorporate too much air), until thermometer registers 185°.
Reduce heat to low and cook milk, without stirring or letting it heat above 185°, for 20-25 minutes (it will be slightly thicker at the longer cook time).
Place pot in a large bowl filled with ice water. Stir constantly until thermometer registers 110° (don’t go below). Immediately and gently stir in yogurt. Pour mixture into a 32-ounce jar. Cover jar and place in a warm spot in kitchen. Let mixture sit for 10-12 hours (do not disturb jar).
Taste yogurt. If you prefer a thicker, tangier yogurt, let it stand 5-8 hours longer. Refrigerate. Remember to reserve 3 tablespoons for your next batch!
I like to add some homemade jam to yogurt, it is amazing meal to start a day, and absolutely natural!