Preheat the oven to 350 degrees F.
Scald the milk: Pour the milk into a medium-sized heavy-bottomed sauce pan (whole milk works best) and heat over medium to high heat just until the first tiny bubbles appear at the edge of the pan.
In a large bowl, whisk together the eggs, sugar, salt, cinnamon, and vanilla. Pour the milk in a slow, steady stream, while whisking rapidly, into the egg mixture.
You may either make one large custard by using a casserole dish or 6 small custards by using a 6-cup muffin pan or individual ramekins or flan cups. Before pouring the custard mixture in the dish or muffin tins, pour the caramel sauce in the bottom of the casserole dish or divide the sauce among the 6 muffin cups (about 2-3 T. per muffin cup). Then carefully pour the custard mixture into the dish or 1/2 c. of the custard mixture into each muffin cup on top of the caramel.
Fill a 9 x 13 pan with an inch or two of hot water and place either the casserole dish or the muffin tin in the middle of it. Place the custard in the water-bath pan in the preheated oven and bake for 45 minutes (only 40 minutes for individual flan custards).