1. Combine butter, cream, salt, sugar, and sweet potatoes in large pot; cook, covered, over low heat, stirring occasionally, until potatoes fall apart when poked with fork, 45 to 55 minutes.
2. Off heat, mash sweet potatoes in saucepan with potato masher, then stir in marmalade and gingerroot . Season with salt and pepper to taste. Spread potato mixture evenly in baking dish. Potato mixture may be made 1 day ahead and chilled, covered. Bring potato mixture to room temperature before proceeding.
In a food processor grind cookies fine. Add the pecans and pulse 3-4 times. Add butter and pulse until mixture resembles soft cookie dough. Wrap topping in wax paper and chill until firm, about 2 hours. Topping may be made 1 day ahead and chilled, covered.