In a heavy bottom saucepan, bring whole milk to a simmer. Stir frequently to avoid burning on the bottom.
Cool whole milk to room temperature (about 70 degrees). Once milk has been cooled, stir in 1/2 cup of buttermilk.
Transfer to a glass bowl and cover. Let this mixture sit at room temperature for 24 hours. The mixture will thicken to a Greek yogurt consistency.
Transfer the mixture again to a cheesecloth-lined strainer over a bowl to catch whey runoff. Place this into the refrigerator so it may drain overnight. The finished consistency will be thick like Greek yogurt and slightly sticky to the touch.
Store in the refrigerator for up to 4 days covered.