In a small pot, simmer cream, milk, sugar and salt until sugar completely dissolves, about 5 minutes. Remove pot from heat. In a separate bowl, whisk yolks. Whisking constantly, slowly whisk about a third of the hot cream into the yolks, then whisk the yolk mixture back into the pot with the cream. Return pot to medium-low heat and gently cook until mixture is thick enough to coat the back of a spoon (about 170 degrees on an instant-read thermometer).