Preheat oven to 325°F (160°C). Place six 6-ounce ramekins in a deep baking dish or roasting pan.

Pour the heavy cream into a medium saucepan. Add the vanilla bean seeds and pod, then heat over medium heat until small bubbles appear around the edges, about 5 minutes. Remove from heat, cover, and steep for 10 minutes. Remove and discard the vanilla pod.

In a large bowl, whisk together the egg yolks, 1/3 cup granulated sugar, lemon zest, and salt until smooth and pale yellow, about 1 minute. Do not overbeat or create foam.

Slowly pour the warm vanilla cream into the egg mixture in a thin stream while whisking constantly. This tempering process prevents the eggs from scrambling.

Strain the custard mixture through a fine-mesh sieve into a large measuring cup or pitcher to remove any cooked egg bits and lemon zest pieces for an ultra-smooth texture.

Divide the custard evenly among the ramekins. Pour hot tap water into the baking dish until it reaches halfway up the sides of the ramekins.

Bake for 35-40 minutes until the custards are set around the edges but still jiggle slightly in the center when gently shaken. The centers should look barely set, not liquid.

Carefully remove the ramekins from the water bath and let cool to room temperature for 30 minutes. Cover each with plastic wrap and refrigerate for at least 4 hours or overnight until completely chilled and firm.

Just before serving, blot any condensation from the custard surfaces with a paper towel. Sprinkle 1 tablespoon of superfine sugar evenly over each custard.

Use a kitchen torch to caramelize the sugar, holding the flame 2-3 inches away and moving in circular motions until the sugar melts and turns deep amber. Let the sugar harden for 2 minutes before serving.