To make the sponge: beat the egg yolks with the sugar until the mixture is light and fluffy. Add softened butter, flour, baking powder and one tablespoon lukewarm water and mix well.
Beat the egg whites with a pinch of salt until stiff. Gently fold into the yolk mixture. Spread out evenly on a Swiss roll tin which has been lined with greased baking parchment. Bake at 180 degrees C for about 12 minutes.
Remove from the oven, let cool for three minutes, then roll up with the parchment. Let cool while you make the buttercream.
To make the buttercream: melt the chocolate in a double boiler. Beat the butter until creamy, then gradually beat in the icing sugar. Beat in the melted chocolate.
To assemble the cake: unroll the sponge, spread evenly with buttercream and roll firmly back up, without the parchment this time! The buttercream will hold it together.
Cut a small diagonal slice off one end to make a branch for the log. Cover the log with buttercream, and use more buttercream to attach the 'branch' to the side. Use a fork in the buttercream to make a bark effect. Dust with cocoa powder and serve!