Preheat oven to 325°F. Place eight 4-ounce ramekins (or six 6-ounce ramekins) in a large roasting pan and set aside. Bring water to a simmer on the stove, enough to halfway-fill the roasting pan later. A tea kettle works great for this.
Mix heavy cream, milk and 1/4 teaspoon sea salt in a medium saucepan. Scrape vanilla bean seeds and add seeds and bean pod to the mixture. Heat just until bubbling then remove from heat. Remove vanilla bean pod. (If using vanilla extract instead of beans, add the extract at step 6.)
In a large heavy saucepan, combine sugar and water. Stir over medium heat until sugar dissolves. Bring to a boil without stirring until mixture turns amber, about 10 minutes.
Remove from heat and carefully trickle in cream/milk mixture while whisking. The mixture will bubble.
Return pan to low heat and whisk vigorously for 3 to 5 minutes, until any hardened sugar clumps dissolve.
Remove from heat. Stir in vanilla extract if not using beans.
In a large bowl whisk egg yolks just until blended. Add about a third of the cream/milk mixture while whisking to temper the yolks. Slowly add the rest of the cream mixture, while whisking the whole time.
Strain the custard through a fine-mesh sieve into a large bowl or pourable measuring cup. Pour evenly into ramekins placed in roasting pan.
Place roasting pan of ramekins in the oven and carefully fill roasting pan with hot water, enough to come halfway up the sides of the ramekins.
Bake 45 to 50 minutes, or until edges of custard are set but center still jiggles.
Cool custards on a wire rack for 45 minutes, then cover and chill in fridge for 4 hours to set.
Sprinkle with flaked sea salt before serving