2 teaspoons citric acid, dissolved into 1/4 cup cool water (if using buttermilk, the citric acid can be reduced to 1 teaspoon)
Place the milk and the buttermilk into a very large pot. Bring the mixture to 75°F. Turn off the heat, cover, and allow the pot to sit undisturbed for 4 hours.
If not using buttermilk, simply heat the milk to 75°F.
Begin to slowly stir in the citric acid, using long, gentle, up-and-down motions. Slowly bring the milk to 195°F, continuing to stir every now and then. Once it has reached temperature you will see the curds begin to form slowly as you stir. Keep stirring, carefully and slowly, until the curds are fully formed and the whey is watery yellow.
Turn off the heat and let the curds stand undisturbed for five minutes to allow all the curds to rise to the top.
Ladle the curds into a colander lined with cheesecloth or butter muslin. Tie the corners into a knot, making a draining bag, and suspend this bag over a bowl to drain for a few hours, or until the queso blanco has reached the consistency you prefer.
Dump the drained cheese into a bowl and salt to taste, if you wish.