Whisk the eggs and sugar in a bowl over a pan of hot water until thickened. Remove from the heat and whisk for 2 minutes. Sift over the flour and cocoa powder, then fold in. Turn into the prepared tin.
Grease and line a 23 cm (9 inch) springform or loose-based cake tin. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes until just firm. Cool on a wire rack.
Sprinkle the gelatine over the measured water in a bowl and leave for 5 minutes. Split the sponge and put one half in the cake tin. Stand the bowl of gelatine in a pan of hot water until the gelatine has melted.
Melt 175 g (6 oz) chocolate over a pan of simmering water (see
Transfer to a serving plate. Remove the greaseproof paper. Melt the remaining chocolate and drizzle it over the top. Chill until ready to serve.