Cook potato in boiling water to cover 20 minutes or until tender; drain. Beat potato at medium speed with an electric mixer until smooth. Stir in sour cream, 1/2 cup butter, 1/2 teaspoon salt, pepper, and, if desired, truffles. Keep warm.
Melt remaining 1/2 cup butter in a skillet over medium-high heat; add green onions, and sauté 2 minutes or until tender. Stir in flour; cook, stirring constantly, 5 minutes. Gradually stir in milk. Stir in salt, red pepper, and wine. Reduce heat, and cook, stirring occasionally, 10 minutes or until thickened.
Stir 1 cup hot mixture into beaten egg yolk; stir into remaining hot mixture. Cook, stirring often, 5 minutes. Add crabmeat, tossing gently.
Pipe or spoon potato mixture evenly into Potato Baskets, if desired; top evenly with crab sauce. Place in bell pepper cups, if desired.