Pour sugar into a flanera, cake pan, or large saucepan if using ramekins.
Set over medium heat and move the pan constantly until the sugar melts and turns deep amber, about 4 to 8 minutes.
Remove from heat and tilt to coat the bottom and sides with caramel, handling carefully. If using ramekins, divide the caramel among them and set aside.
In a heavy saucepan, combine whole milk, evaporated milk, sugar, cinnamon stick, vanilla bean or extract, lemon zest, and salt.
Bring to a boil over medium heat, then remove from heat and let cool to room temperature, about 30 minutes.
Preheat the oven to 350°F (177°C).
Discard the cinnamon stick, vanilla bean, and lemon zest.
Whisk the cooled milk mixture with eggs until well combined.
Pour through a strainer if desired, then transfer to the caramel-coated pan or ramekins.
Cover tightly with a lid or aluminum foil.
Set the pan or ramekins inside a larger roasting pan and pour in enough hot water to reach halfway up the sides.
Bake for 60 to 75 minutes until a knife inserted in the center comes out clean.
Cool completely at room temperature, then refrigerate for at least 4 hours or overnight.
To unmold, run a thin knife around the edges, shake gently, then invert onto a plate.
The caramel will spill over, creating a luscious sauce.
Serve immediately.