Heat sugar in a saucepan over medium heat until it melts into a deep amber caramel. Swirl the pan gently until smooth and golden.
Quickly pour caramel into your baking dish or flanera and tilt to coat the bottom. It will harden fast as it cools.
In a blender, combine eggs, condensed milk, evaporated milk, vanilla, and salt. Blend until smooth and slightly frothy.
Pour the custard mixture over the hardened caramel. It should look silky and even.
Place the dish inside a roasting pan and add hot water halfway up the sides for a water bath.
Bake at 350°F (175°C) for about 60 minutes until edges are set but center still jiggles slightly.
Remove and cool to room temperature, then refrigerate for at least 4 hours or overnight until fully set.
Run a knife around the edges and invert onto a plate so caramel flows over the top.