Preheat the oven to 180°C (350°F/Gas 4). Grease a 20 x 30 cm (8 x 12 inch) baking tin and line with baking paper. Line a baking tray with baking paper.

Sift the flour, baking powder and cocoa. Cream the butter and 145 g (51?4 oz/2/3 cup) sugar until pale and fluffy. Beat in the egg yolks and the vanilla. Fold in the flour mixture and the milk. Beat the egg whites in a clean bowl using electric beaters until light and foamy. Add the remaining sugar to the egg whites and beat until soft peaks form. Stir a tablespoon of the whisked egg white into the sponge mixture, then gently fold in the rest. Pour the mixture into the prepared tin and bake for 25–30 minutes or until firm. Allow to cool for

5 minutes before turning out onto a wire rack.
Beat the cream, sugar and rum in a bowl using electric beaters until firm peaks form. Remove one-third of the cream, cover with plastic wrap and refrigerate. Gently fold the morello cherries through the remaining cream.
Slice the sponge horizontally into three layers. Place the first layer on the prepared baking tray and spread with half the cherries and cream mixture. Top with another layer of sponge, and the remaining cherries and cream mixture. Finish with the remaining slice of sponge and refrigerate for 2 hours, or until firm.

Cut the sponge into 3 cm (11?4 inch) squares. Use a flat-bladed knife to spread one-quarter of the reserved cream around the outside of the petits fours. Coat the outside of the petits fours with the grated chocolate. Spoon cream on top of each petit four and top with a maraschino cherry. Serve chilled.