In a 5-quart saucepan, combine milk, cream, sugar and kosher salt. Place on stovetop over medium heat; stirring occasionally with a silicone or other heat resistant spatula. The mixture should begin to simmer at about 12 minutes, if much faster, reduce heat slightly; much slower, increase heat slightly.
Stirring constantly, simmer for another 30 minutes, scraping the sides often. When the mixture has reduced by about half it will suddenly turn foamy. Keep stirring until the foam subsides and pour mixture into a glass measuring pitcher. You should have 2 cups, if less, you can top with milk and blend to emulsify. If you have more than two cups, return to pan and continue to simmer until reduced to 2 cups.
Store in a sealed, airtight container in the refrigerator for up to a month. Bring to room temperature before using.
Video
Notes
This makes about 2 cups, or 16 2-tablespoon servings.