Preheat the oven to 320°F (160°C).
In a large saucepan, combine the full cream milk, evaporated milk, and salt.
Split the vanilla bean lengthwise using a sharp knife. Scrape out the seeds and add both the seeds and pod to the milk mixture.
Heat over low heat until the mixture is hot and just beginning to bubble around the edges. Remove from heat.
In a separate mixing bowl, whisk together the egg yolks and caster sugar for about 1 minute until smooth and slightly pale.
Slowly pour the warm milk mixture into the egg yolks while whisking continuously to temper the eggs.
Place 4 ramekins into a deep baking dish.
Pour the custard mixture evenly into the ramekins.
Carefully pour boiling water into the baking dish until it reaches about one-quarter of the way up the sides of the ramekins.
Bake for 30–40 minutes, until the custards are set around the edges but still slightly jiggly in the center.
Remove the ramekins from the water bath and allow them to cool completely.
Refrigerate for at least 2 hours before serving.
Just before serving, sprinkle about 1 teaspoon of caster sugar evenly over each custard.
Use a kitchen torch to caramelize the sugar until golden and crisp.
Let the sugar harden for 1–2 minutes, then serve.