1.Preheat oven to 180°C/350°F. Grease a 20cm x 30cm (8-inch x 12-inch) rectangular pan; line base and sides with baking paper, extending the paper 5cm (2 inches) over sides.
2.Beat butter, sugar and grated rind in a small bowl with electric mixer until light and fluffy. Beat in eggs, one at a time. Stir in juice and sifted flours. Spread mixture into pan.
3.Bake cake about 35 minutes. Stand in pan 10 minutes; transfer to a wire rack to cool. Using a 3.5cm (1½-inch) cutter, cut 28 rounds from cooled cake. Discard excess cake.
4.Bring cream to the boil in a small saucepan. Remove from heat; pour over chocolate in a small bowl, stir until smooth. Stand about 20 minutes or until thickened.
5.Slice strips of rind thinly; place in a small bowl with the boiling water. Soak 30 seconds; drain. Transfer rind to a small bowl of iced water; drain.
6.Place cake rounds on a wire rack over a baking-paper-lined tray; spread ganache on cakes, top with rind. Stand until set.
You can serve petit fours in patty paper cases. You could use candied orange peel instead of the thinly sliced orange rind. Petit fours are best made on the day of serving.Note