Heat 1/2 gal whole milk to 180 F on stovetop, stirring frequently to prevent milk from sticking on the bottom of the pan. Cool in ice bath to 110 F.
Thaw out 2 starter pucks* from freezer and mix with a cup of the warm milk until smooth. Add to the rest of the milk and mix thoroughly.
Pour into jars and sous vide at 109 F for 5 hours or longer.
To strain, pour into colander lined with cheesecloth and set over bowl to catch the whey. Strain overnight. If yogurt is too thick, add some whey back in and mix well. Spoon into jars and refrigerate.
*I spooned my last batch of Bulgarian yogurt into a silicon muffin pan and froze. I pooped them out and stored in a ziplock baggie in the freezer. Two of them is enough starter for 1/2 gallon of milk.