Adjust the oven rack to the middle position. Place a fine-mesh strainer over a large bowl, then set the bowl in a larger bowl filled with ice water. Heat the milk in a large saucepan over medium-low heat (do not stir while heating) until it registers 185 degrees F.
Remove the pot from the heat, gently stir in the milk powder, and let it cool to 160 degrees F, approximately 7-10 minutes. Strain the milk through the prepared strainer and let it cool, gently stirring occasionally, until the milk registers 110-112 degrees F. Remove from the ice bath.
In a small bowl, gently stir 1/2 cup warm milk into the greek yogurt until smooth. Stir the yogurt mixture back into the milk. Cover tightly with plastic wrap and poke several holes in the plastic.
Place the bowl in the oven and turn on the oven light, creating a warm environment of 100-110 degrees F. Let the yogurt sit undisturbed until thickened and set, 5-7 hours. Transfer to refrigerator until completely chilled, about 3 hours.
Set a clean fine-mesh strainer over a large measuring cup and line it with a double layer of coffee filters. Transfer the yogurt to the prepared strainer, cover with plastic wrap, and refrigerate until about 2 cups of liquid have drained into the measuring cup, 7-8 hours. (You can also skip this step for regular yogurt that is less thick). Yogurt can be refrigerated for up to one week.
Reprinted with permission from 'The America's Test Kitchen DIY Cookbook' by the editors at America's Test Kitchen