• Prep: Grease an 8½x5½x3-inch loaf pan with cooking spray, and line tightly with foil.
• Semifreddo: Heat butter or margarine and chocolate in a medium saucepan over low heat.  Stir until melted and mixture is uniform and smooth; remove from heat and set aside to cool. 
• Meanwhile, combine egg yolks and sugar in the bowl of a standing electric mixer. Beat on high until sugar is dissolved and mixture is thick and pale yellow in color. Add chocolate mixture, mixing until combined. Set aside.
• In a separate large bowl, beat egg whites on high until stiff peaks form (avoid beating to dry point). Using a rubber spatula, gently fold egg whites into chocolate mixture (this can be done in batches) until combined. Pour into the prepared loaf pan and freeze (at least 6 hours), tightly covered.
• Serve: Remove semifreddo from freezer and turn out onto plate; peel off foil. Cut into ¾-inch thick slices and place on individual plates. Spoon warm orange sauce over slices and serve immediately.

Candied Orange Sauce

• Combine all ingredients in a medium saucepan over medium heat. Cook, stirring often, until sauce begins to boil and thicken, about 5 minutes. Cool slightly.
• Plan Ahead - Semifreddo can be prepared up to 2 weeks ahead and stored in the freezer. Candied orange sauce can be made up to 2 days ahead. Store in refrigerator, covered well.  Rewarm before serving.