Line a 2-quart square baking dish with foil, extending foil over edges of dish. In a medium bowl stir together the butter and biscotti crumbs. Spread evenly in the prepared baking dish. Press firmly to form an even crust. Place in the freezer.
In a chilled bowl stir ice cream to soften. In another bowl stir mascarpone cheese to soften. Slowly add the softened vanilla ice cream to the mascarpone, stirring well after each addition (mixture may look slightly lumpy). Spoon into crust-lined dish and spread evenly with the back of a large spoon. Freeze about 1 hour or until firm. Spoon sorbet on top of the vanilla ice cream and spread evenly with the back of a large spoon. Cover and freeze 4 hours or until firm.
Let frozen mixture stand at room temperature for 10 minutes. Lift from pan using foil. In a small saucepan cook and stir marmalade just until heated through. Stir in citrus sections.
To serve, cut ice cream mixture into 9 squares. Top each square with some of the citrus mixture. Makes 9 servings.