Preheat oven to 400°F. Place sweet potatoes on a foil-lined sheet pan and pierce several times with a fork. Bake 1 hour, or until tender. Cool enough to handle, then peel and mash well.

Reduce oven temperature to 350°F. Make the streusel by blending brown sugar, softened butter, flour, and chopped pecans together with a pastry blender or fork.

Mix mashed sweet potatoes with brown sugar, eggs, butter, and vanilla. Transfer to a lightly buttered 9 x 9-inch casserole dish.

Sprinkle with streusel topping and bake at 350°F for 30 minutes, or until the casserole is set.