Trim edges of cake and cut into two 10-inch long strips.
TO MAKE GRAND MARNIER CREAM, combine butter, powdered sugar and orange peel in a medium bowl and beat until soft and smooth. Add egg yolk and Grand Marnier in small amounts until thoroughly blended. If mixture separates, place in a bowl set in warm water, and whisk with a fork until smooth. Chill until cream will hold a shape.
Spread top of cake with Grand Marnier cream. Place remaining cream in a piping bag with a star tube and decorate cake. Chill cake until cream is hard. Cut each strip of cake into 10 pieces. Place cakes on a wire rack set over a jelly roll pan. Spoon icing over cakes.
TO MAKE ICING, cook corn syrup and water in a medium saucepan until corn syrup has melted and water is just bubbling. Stir in Grand Marnier and powdered sugar. Remove from heat and beat until smooth. Reheat until tepid. If icing is too thick, add 1 teaspoon of water. Tint icing with yellow food coloring. Spoon icing over cakes. Let stand until icing is set. Decorate with angelica strips. Makes 20 petits fours.
TO MAKE GENOISE: Preheat oven to 350°F Grease and flour bottom and sides of a 14 inch x 10 inch baking pan. Shake out excess flour.
Combine eggs and sugar in a medium mixing bowl. Place bowl in a pan of hot water. Beat until mixture is very thick and just warm. Mixture should form a ribbon when beaters are lifted. Remove from heat and add vanilla. Continue beating until mixture is almost cool. Fold flour into egg and sugar mixture. Mix in butter.
Pour batter into prepared greased pan; level top. Bake in a preheated oven 20-25 minutes or until set on top. Cool pan on a wire rack 10 minutes. Run a knife carefully around the edge. Remove from pan. Cool completely on wire rack. Use within 36 hours or wrap and freeze up to 6 weeks.