For the sponge
- butter, for greasing
- 7 medium free-range eggs, separated
- 140g/5oz caster sugar
- 65g/2¼oz cocoa powder
For the filling
- 200ml/7fl oz double cream
- 75ml/2½fl oz crème fraiche
- 100g/3½oz raspberries
- 40g/1½oz dark chocolate
- Preheat the oven to 180C/160C Fan/Gas 4. Line a 22x32cm/9x13in Swiss roll tin, or tall-sided baking tray with baking paper and lightly grease with butter.
- Whisk the egg yolks and sugar together in a bowl until thick. Sieve over the cocoa powder and whisk in until full incorporated.
- In a clean glass bowl using clean beaters, whisk the egg whites until stiff. Fold them into the cocoa mixture a spoonful at a time. Pour the mixture into the baking tin and spread out evenly. Bake for 20 minutes. Use a knife to cut the edges away from the tin, then leave to cool.
- Turn the Swiss roll out on to a sheet of baking paper. Whisk the double cream, then fold in the crème fraiche. Using a spatula, spread this over the sponge. Sprinkle over the raspberries. Roll up from one of the short sides.
- Melt the dark chocolate in a microwave or in a bowl over a pan of boiling water. Drizzle the chocolate over the top of the cake.