Preheat oven to 325°F (165°C). Place four 6-ounce ramekins in a deep baking dish and set aside. Bring a kettle of water to a boil for the water bath.

Heat the coconut milk in a medium saucepan over medium heat until it just begins to steam and small bubbles form around the edges, about 4-5 minutes. Do not let it boil. Remove from heat.

In a medium bowl, whisk together the egg yolks, 1/3 cup sugar, vanilla extract, lime zest, and salt until the mixture is pale and slightly thickened, about 1 minute.

Slowly pour the hot coconut milk into the egg mixture in a thin stream, whisking constantly to temper the eggs. Add the first few tablespoons very slowly to prevent curdling.

Strain the custard through a fine-mesh sieve into a large measuring cup or pitcher to remove any cooked egg bits and the lime zest pieces. This ensures a perfectly smooth texture.

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

Carefully transfer the baking dish to the oven and bake for 35-40 minutes, until the edges are set but the centers still jiggle slightly when gently shaken.

Remove the ramekins from the water bath and let cool to room temperature, about 30 minutes. Cover with plastic wrap and refrigerate for at least 4 hours or overnight until completely chilled and set.

Just before serving, blot any condensation from the custard surfaces with a paper towel. Sprinkle 1-1½ tablespoons of sugar evenly over each custard in a thin layer.

Using a kitchen torch, hold the flame 2-3 inches from the surface and move it in circular motions until the sugar melts and caramelizes to a deep amber color. Let the sugar harden for 1-2 minutes, then garnish with toasted coconut if desired and serve immediately.