Preheat oven to 325°F. Place four 6-oz ramekins in a deep baking dish. Bring a kettle of water to a boil.
In a small saucepan, heat the heavy whipping cream over medium heat until it just begins to steam and tiny bubbles form around the edges. Do not let it boil. Remove from heat.
In a mixing bowl, whisk the egg yolks, powdered erythritol, vanilla extract, and sea salt together until pale and smooth. Very slowly pour the hot cream into the egg mixture in a thin stream, whisking constantly to temper the eggs without scrambling them. Strain the custard through a fine mesh strainer for an ultra-smooth texture.
Divide the custard evenly among the four ramekins. Pour boiling water into the baking dish until it reaches halfway up the sides of the ramekins. Carefully transfer to the oven and bake for 40 to 45 minutes, until the custards are set around the edges but still have a gentle wobble in the center.
Remove the ramekins from the water bath and let cool to room temperature, then refrigerate for at least 4 hours or overnight until completely chilled and set.
When ready to serve, sprinkle 1 teaspoon of granular erythritol evenly over each custard. Use a kitchen torch to caramelize the surface until golden and crackly. Let the topping harden for 30 seconds, then serve immediately.
