Step 1

Preheat the oven to 170°C. Line the base of a 38 x 26 cm swiss roll tin with baking paper. You will need a few dressmaking pins to secure the cake.

Step 2

Melt the chocolate with the coffee and water in a double boiler or in a heatproof bowl over a saucepan of simmering water. Allow to cool slightly. Beat the egg whites until stiff peaks form. In a separate bowl, beat the egg yolks with the sugar until pale and fluffy, then beat in the melted chocolate. Fold this mixture into the egg whites and spread it out into the prepared tin.

Step 3

Bake for 20 minutes, or until a skewer inserted into the cake comes out clean. When the cake is cool, cover with a tea towel and put in the fridge for 1 hour.

Step 4

Beat the cream with the icing sugar and vanilla seeds until thick, and set aside.

Step 5

Remove the cake from the fridge. Sift the cocoa powder over the cake, then cover with a piece of baking paper and turn it over. Remove the tin and the baking paper that was on base of the cake.

Step 6

Place cream in the middle third of the cake, along its full length, and make a large long log as follows. Lift up the two long sides of the cake and bring them over to meet in the middle, sliding one side under the other to encase the cream. Using dressmaking pins to secure the package, join the paper and fold down, holding the paper tightly around the log to help hold its shape. Put the cake in the fridge until ready to serve.