Combine the milk and buttermilk in a large pan over medium heat. Cook, stirring occasionally, until the mixture measures 170 degrees F on a candy thermometer. Adjust the heat so the temperature stays between 170-175 degrees F. Let the mixture cook for 30 minutes or until the solids and liquid starts to separate (curds and whey). Line a strainer with several layers of cheesecloth and place the strainer over a large bowl. Spoon the curds from the pan into the lined strainer then pour the remaining liquid over the curds. Let the mixture drain for 2-4 hours at room temperature. Transfer the drained curds to a blender or food processor. Add the salt and process until smooth and creamy. Store the cream cheese in a tightly covered container in the refrigerator for up to 10 days.