Steam sweet potatoes until tender.
Preheat the oven to 325ºF. Lightly butter six ramekins.
Bring the milk, sugar, and vanilla bean just to the boil, then turn off the heat. Stir to make sure the sugar is dissolved. Scrape the vanilla seeds into the milk, and return the pods to the pan. Let stand for the flavor to infuse for 15 minutes.
Scoop the flesh from the sweet potato and measure 3/4 cup. Puree with the eggs, it will have a somewhat sticky texture. Add the milk and process just until well blended. You want a minimum amount of bubbles for a smooth custard.
Pour the liquid into the ramekins and set them in a large pan. Add boiling water to come halfway up the sides of the ramekins. Bake until set but just a bit wobbly at the very center, about 45 minutes. Leave the custards in the water bath until ready to serve. Run a knife along the edge of the cups, then turn them out onto serving plates.
Drizzle with warm chestnut honey.