1. Combine butter, cream, salt, sugar, and sweet potatoes in 3- to 4-quart saucepan; cook, covered, over low heat, stirring occasionally, until potatoes fall apart when poked with fork, 35 to 45 minutes.
2. Off heat, mash sweet potatoes in saucepan with potato masher, or transfer mixture to hopper of food mill and process into warmed server bowl. Stir in pepper; serve immediately.
Cutting the sweet potatoes into slices of even thickness is important so that they cook at the same rate. A potato masher will yield slightly lumpy sweet potatoes; a food mill will make a perfectly smooth puree. The potatoes are best served immediately, but they can be covered tightly with plastic warp and kept relatively hot for 30 minutes. This recipe can be doubled and prepared in a Dutch Oven; the cooking time will need to be doubled as well.