Cook sweet potatoes in a Dutch oven in boiling water to cover over medium heat 30 minutes or until tender. Cool; peel and mash potatoes.
Combine mashed sweet potato, 1/2 cup brown sugar, and next 4 ingredients in a mixing bowl; beat at medium speed with an electric mixer until smooth. (Or mash with a potato masher until smooth.) Spoon into a greased 2 1/2-quart or 13" x 9" baking dish.
Combine 1/2 cup brown sugar and flour. Cut in 1/4 cup cold butter with a pastry blender until crumbly. Stir in crushed gingersnaps. Sprinkle streusel over sweet potato. Bake, uncovered, at 350° for 25 minutes or until streusel is lightly browned.
Make Ahead: Prepare sweet potato filling, and spoon into a greased 2 1/2-quart microwave-safe dish. Cover and chill overnight. Microwave at HIGH 10 minutes or until hot. Prepare streusel, and sprinkle over filling. Bake, uncovered, at 350° for 20 minutes or until streusel is lightly browned.
Purchase crisp gingersnap cookies rather than the soft variety for making the streusel.