Preheat the oven to 190℃ (180℃ fan)/400°F/gas mark 5. Grease and line a 25x38 cm (10x15in) Swiss roll tin with baking parchment.
Place 6 eggs and 175g (¾ cup) caster sugar in a mixing bowl and whisk until very thick and creamy. The mixture is ready when it leaves a trail for about 10 seconds after lifting out the whisk.
Sift in 100g (¾ cup) flour and 3 tablespoon cocoa powder over the whisked eggs and carefully fold in with a spatula or metal spoon until evenly combined.
Turn the mixture into the prepared tin and gently tip the pan so that the mixture goes into the corners. Bake in the oven for 15 minutes or until springy to the touch.
Turn the cake out onto another sheet of baking parchment lightly sprinkled with caster sugar. Carefully remove the lining paper. Trim off the edges and cut a knick halfway through the cake about 2cm (1in) from the short end (or long end if you are making a long thin log).
Fold the indented end in and roll up tightly with the baking parchment inside the roll. Place on a wire rack and allow to cool completely.
Meanwhile, make the icing. Melt 200g (7oz) chocolate in a bowl over a pan of hot water, stirring until fully melted. Remove from the pan and allow to cool.
Beat 250g (9oz) butter and 250g (2⅛cup) icing sugar together until pale and fluffy, then gradually beat in the cooled chocolate.
To complete the cake. Carefully unroll the Swiss roll and spread with about one-third of the buttercream. Carefully re-roll without the paper. Cut off a wedge from one end and place the cake on a plate or board with the cut off piece butting up to one side.
Carefully spread the remaining chocolate icing all over the cake. Use a fork to make a bark effect along the lengths of the log and use a small knife to spread the chocolate in a circular movement over the ends of the log.
Chill for about 1 hour before decorating with a dusting of icing sugar.
Notes
I have made this with 70% cocoa solids but my family found this a bit too rich. We like it best with a plain chocolate of around 54%.Freeze: For up to 3 months. Thaw overnight in a cool room.The cake will keep for up to 1 week if stored in an airtight container in a cool place. Best served at room temperature.Nutrition information is calculated usingan online nutrition database - is approximate and is meant as a guideline only.