Preheat your oven to 325°F. Place 4 (5 ounce) ramekins in a large roasting pan. Set aside.
Make the caramel sauce: Fill a medium sauce pan with water and sugar. Place the pot over medium-high heat and heat the mixture, stirring occasionally, until the sugar is dissolved. Bring the mixture up to a boil, and boil for abut 10 minutes, or until the sugar turns a honey brown color. Do not mix once the sugar is dissolved!! * (see notes) If needed, swirl the pan gently.
Once the caramel is honey colored, remove from the heat and continue swirling the pan until it turns a deep amber color (about 10 seconds). Quickly (but carefully!) pour out 2 to 3 tablespoons of caramel sauce into the bottom of each ramekin. Swirl the ramekin to even out the sauce and cover the corners (it doesn't have to be perfect!). The sugar will harden, but that's perfectly normal. Set aside to make the custard layer.
Make the custard: In a large bowl, combine the eggs, sugar, salt, and vanilla. Whisk together until the eggs are broken up and the sugar is dissolved. Add the milk and whisk to combine.
Strain the custard to remove any pieces of egg that didn't get incorporated. Pour the strained custard into the ramekins (over the caramel sauce) until just under filled to the top.
Fill the roasting pan with hot water going halfway up the sides of the ramekins to create a water bath for the flan.
Place the roasting pan in the center of your preheated 325°F oven. Bake for 40 to 45 minutes. To check if your flan is done, insert a thin knife into the center of one of the flans (halfway down). The knife should come out clean, but the flans should still jiggle. The top should also be dry to the touch, but soft.
Take the roasting pan out of the oven, and carefully remove the flans from the water bath. Let the flans cool completely to room temperature. Cover, and refrigerate for at least 4 hours or overnight (preferable). Once chilled, run a knife along the edges of your flan, and place a plate (with a lip! ** see notes) over the ramekin upside down. Quickly flip the flan and the plate. Lift up the ramekin to reveal the flipped flan.