Preheat oven to 325F/170°C
Mix 1 1/2 cups sugar with 1/4 teaspoon salt and 1/4 cup water in a heavy bottomed sauce pan over medium heat. Continue to stir while the sugar liquifies and bubbles and starts to caramelize (about 5 minutes). Due to the sugar’s original dark color, it’s very hard to judge the caramelization visually. Just stir the sugar until you can smell a nice toasty aroma, but be careful that it does not burn.
Remove the pan from heat and add the cream and milk slowly, whisking until the sugar is completely mixed with the liquids. Set aside.
With a whisk or hand mixer, blend egg yolks with 1/2 cup dark brown sugar and vanilla.
Slowly add the caramel/cream mixture into the egg mixture, while continuing to blend with a whisk. Next, strain the liquid through a fine mesh sieve to remove any bits of scrambled egg.
Place the pots in a roasting pan or large oven-proof skillet. Fill hot water half-way up the sides of the pots. Tightly cover with aluminum foil, poke a few holes in the foil, and bake at 325 degrees for about 40 minutes until the custard is set along the sides but the middle is still slightly wobbly. Remove the pan from the oven, then remove the pots from the water and let cool to room temperature. Refrigerate, covered, for a minimum of one to two hours. Garnish with flaky sea salt right before serving.