While any sweet potatoes work, bright-colored Garnets are particularly good in this gratin. If the potatoes are stringy, as some varieties can be, purée the flesh in a food processor prior to whipping with the cream.

Steps

1. Adjust oven rack to lower-middle position and heat oven to 400 degrees. Poke sweet potatoes several times with a paring knife and spread potatoes on a rimmed baking sheet. Bake, turning once, until very tender and easily pierced with a knife, 1 to 1½ hours. Cool (cutting in half will expedite cooling) until easily handled. Scrape flesh from skins into a large mixing bowl. (See Note.)
2. Increase oven temperature to 425 degrees. Combine cream, bay leaf, and a generous pinch of salt in a small saucepan. Using a sharp paring knife, split vanilla bean in half lengthwise and scrape seeds into the cream. Add vanilla pod to the pan and bring to a simmer; cover, remove from heat, and steep for at least 10 minutes. Remove pod and bay leaf.
3. Using a standing or hand-held electric mixer, whip potatoes, infused cream, 1 tablespoon brown sugar, and a liberal amount of black pepper until smooth, light, and homogenously blended. Adjust seasoning to taste and evenly spread potato mixture into a 7-by-11-inch (or 9-by-13-inch) Pyrex or oven-safe baking dish. Sprinkle remaining brown sugar in an even layer across the top of the potatoes. Bake until bubbling around the edges, about 20 minutes.
4. Adjust oven rack as close as possible to broiler element and broil gratin until the sugar is well browned, bubbling, and crispy, 4 to 10 minutes. Serve immediately.