Ancho Sweet-Potato Purée

Preheat oven to 450 °F. and line a baking sheet with foil. Prick sweet potatoes in several places with a fork and bake on baking sheet in middle of oven until tender, about 1 hour. Cool potatoes until they can be handled. Wearing rubber gloves, stem and seed chiles and tear into 1-inch pieces. In a small saucepan bring chiles and cream to a boil. In a blender purée mixture until smooth (use caution when blending hot liquids). Scoop flesh from potatoes into a food processor, discarding skins, and purée with cream mixture, salt, and 2 tablespoons butter. Purée may be made up to this point 1 day ahead and cooled before being chilled, covered. Transfer purée to a large saucepan and heat over moderate heat, stirring, until hot, stirring in more salt and butter if desired.