Heat olive oil in a pan over medium heat, and saute the onion and garlic until translucent. Stir 2 tablespoons cornstarch into 2 tablespoons chicken stock to make a slurry. Add the remaining chicken stock to the onion/garlic mixture and bring to simmer. Add the slurry and bring to boil. When the mixture comes to a boil, remove from the heat. Add the salt, pepper, tarragon, and yogurt and heat until warmed through, but do not boil, about 1 minute.
Pour milk into small saucepan and whisk in powdered milk and honey. Place over medium heat and bring to 120 degrees F on an instant read thermometer. Once milk has reached 120 degrees F, pour into a cylindrical plastic container, reserving 1/2 cup. Whisk in the reserved 1/2 cup into the yogurt and add back to the milk mixture.
Place container into a narrow wine bucket, lined with a heating pad. Set the heating pad to medium. Let the mixture ferment for 3 to 12 hours making sure the temperature stays as close to 115 degrees F as possible.
After fermentation is complete place into the refrigerator overnight.
Recipe courtesy of Alton Brown