Add the sugar and water to a heavy saucepan over medium heat. Do not stir, only swirl the pan.
Cook 5 to 7 minutes, until the sugar turns a light amber color.
Remove from the heat and carefully whisk in the warm cream, butter and salt.
Let the sauce cool for 10 minutes to thicken slightly.
Build the Lattes
Stir 1 tablespoon of caramel sauce into the bottom of each mug.
Brew the espresso over the sauce and stir until blended.
Warm the milk to about 150 degrees F and whisk until foamy.
Pour the milk into the mugs, holding back the foam.
Spoon the foam on top and drizzle each latte with 1 tablespoon of caramel sauce.
Notes
Do not stir the sugar while melting. Warm the cream before adding. Leftover sauce keeps 2 weeks in the fridge. For an iced latte, pour sauce over ice, then cold milk and espresso.