2 teaspoons citric acid, dissolved into ¼ cup cool water (if using buttermilk, reduce citric acid to 1 teaspoon)
Pour milk and buttermilk into a large pot. Bring the mixture to 75°F. Turn off heat. If using buttermilk, cover, and allow the pot to sit undisturbed for 4 hours.
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 heat and let curds stand undisturbed for 5 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 over a bowl to drain for a few hours, or until queso blanco has reached the desired consistency.
Transfer the drained cheese into a bowl and salt to taste. Store in refrigerator up to 2 weeks.