Scrub the sweet potatoes, wrap in aluminum foil and bake until soft at 425 degrees (may take over 1 hour). Peel off the outer skin and mash the potatoes. Combine the mashed sweet potatoes, granulated sugar, sour cream, beaten eggs, vanilla extract, and 1/2 cup melted butter. Spread in an 8x8-inch casserole dish. Mix the brown sugar, flour, chopped pecans, and 1/3 cup cold butter, cutting in butter until crumbly and sprinkle on top of casserole. Bake at 350 degrees for 30 minutes.