Preheat the oven to 325 degrees F.
Heat the milk, cream, sugars, espresso, vanilla and salt in a medium saucepan over low heat until just boiling. Remove from heat. Whisk the egg and yolks in a measuring cup and temper by mixing in a small amount of the cream mixture into the eggs before adding the rest of the cream mixture.
Pour into 4 (4-ounce) ramekins and arrange on a baking dish. Fill the baking dish halfway with very hot water but not boiling, being careful not to splash water into the ramekins. Put in the oven and bake until the edges are set, but the center is still a little wobbly, about 20 to 25 minutes. Remove immediately from the water bath to a baking dish filled with ice to chill quickly. Keep refrigerated until serving.