Adjust the oven rack to the middle position. Preheat the oven to 160°C. Spray the inside of 2 ramekin dishes or teacups lightly with a non-stick spray.
Melt the sugar in the water using a small saucepan, work over medium-high heat. As the sugar caramelizes, you must not stir, but shake and tilt the pan to incorporate all the dry sugar. Keep the heat to medium, so that the sugar does not burn, and continue cooking until you have a clear deep amber liquid. Remove the pan from the heat and immediately pour equal amounts of caramel into the ramekins or cups, tilting to distribute the caramel evenly, and allow to cool slightly until hardened.
Boil the kettle in readiness for the water bath. In the same saucepan away from the heat, thoroughly whisk together the milk, condensed milk, egg, egg yolk and salt. Divide this custard between the two ramekins or cups. Line an oven roasting pan with a clean drying up cloth and place the dishes on the cloth. Add boiling water to come halfway up the sides of the ramekins or cups. Place the pan in the oven. Bake the custards until the centres are just barely set, ± 15 minutes. Carefully remove the ramekins from the water bath and allow the custards to cool to room temperature, about 1 hour. Cover tightly with plastic wrap and refrigerate until cold, at least 2 hours or overnight.
Run a small knife around the inside of the ramekins to loosen the custard. Place an inverted serving plate over the top and quickly flip the custard onto the plate, drizzling any extra caramel sauce over the top (some caramel will remain stuck in the cups). Repeat with the remaining custard.