TO MAKE TOPPING, press jam through a fine strainer into a small saucepan; stir in water. Cook, stirring, 1 to 2 minutes or until mixture is smooth. Cool slightly. Brush over top of cake.
Roll out marzipan thinly between 2 sheets of waxed paper. Remove 1 sheet of waxed paper from marzipan. Invert marzipan on top of cake; peel off waxed paper.
Trim marzipan edges even with cake. Chill 30 minutes.
Using a very sharp knife, cut cake into 1-inch squares and chill to firm. Place on a wire rack set over a baking sheet.
If making round petits fours use a pastry cutter to cut circles of cake. Brush top and sides with topping. Cut circles of marzipan with pastry cutter. Place on top of cake. To coat sides, roll on strip of marzipan.
TO MAKE ICING, combine corn syrup and 2 Tablespoons water in a bowl or top of a double boiler set over a pan of simmering water. Add powdered sugar and mix well. Add 1 to 2 more Tablespoons of warm water. Warm until mixture is smooth. Tint with food coloring, if desired. Carefully spoon icing over marzipanned cake. Leave to set. Decorate with flowers or fruit or pipe with melted chocolate.
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.
TO MAKE MARZIPAN: Combine ground almonds and sugar in a medium bowl. Add lemon juice, sherry or brandy, and almond extract, if desired.
Gradually mix in enough egg white to ensure that paste is sticky, but not wet.
Knead paste until smooth on a pastry board dusted with powdered sugar. Wrap in plastic wrap. Store in refrigerator up to 4 weeks.