Preheat oven to 325°F (163°C) and position rack in center. Place ramekins in a large roasting pan and boil water for the water bath. Set a fine-mesh strainer over a large measuring cup with a pour spout.
Pour heavy cream into a medium saucepan. If using a vanilla bean, split lengthwise, scrape out seeds, and add both seeds and pod to the cream. Heat over medium-low until hot with small bubbles around the edges, do not boil. Remove from heat and let sit 10-15 minutes if using vanilla bean. Remove and discard pod.
Separate egg yolks into a medium bowl. Add 1/3 cup sugar and salt. Whisk vigorously for 1-2 minutes until pale yellow and slightly thickened, with sugar mostly dissolved.
Slowly pour about 1/4 cup of warm cream into the egg yolk mixture while whisking constantly to temper the eggs. Continue adding remaining cream in a slow stream while whisking. If using vanilla extract instead of bean, stir it in now.
Pour custard mixture through the fine-mesh strainer into the measuring cup. Carefully pour into each ramekin, filling about 3/4 full. Skim off any bubbles from the surface with a spoon or torch.
Place roasting pan with filled ramekins in the oven. Carefully pour hot water into the pan until it comes halfway up the sides of the ramekins. Bake for 35-45 minutes until edges are set but centers still have a slight wobble when shaken.
Carefully remove roasting pan from oven. Use tongs to lift each ramekin out and place on a cooling rack. Cool at room temperature for 30 minutes, then cover with plastic wrap and refrigerate for at least 4 hours or up to 3 days.
Just before serving, remove custards from refrigerator. Blot any condensation with a paper towel. Sprinkle 1-2 teaspoons of sugar evenly over each surface, tilting to distribute and tapping out excess.
If using a kitchen torch, hold it 2-3 inches from the surface and move in circular motions until sugar melts and turns golden brown, about 1-2 minutes per ramekin. If using broiler, place on top rack 4-5 inches from element and broil 2-4 minutes, watching constantly.
Let caramelized tops cool for 2-3 minutes to harden into a crispy shell before serving. Enjoy the satisfying crack of the sugar top and the creamy custard beneath.