In a bowl whisk eggs and sugar until pale and fluffy, thick enough to leave a substantial trail with the whisk.
Sieve flour and cocoa powder together. Fold in gently into egg mix using a metal spoon or spatula.
Pour mixture into tin.
Bake for 8-12mins until springy to touch and slightly shrunk from edges.
Turn out onto a sheet of baking parchment and trim off edges. Lay another sheet on top and roll up (empty). Set aside to cool.
Meanwhile whip cream & blend in liqueur for filling.
Unroll cake and remove paper. Spread upper surface of cake with cream and re-roll, with join placed on underside to hold in place.
For butter icing, beat sugar and cocoa into butter, gradually until well blended, adding a little hot water from the kettle – only a tsp at a time – if the mixture is too stiff.
To complete cake, cut obliquely and arrange like a log.
Spread liberally with icing and mark with a fork to resemble bark. Dust with icing sugar.