Pre-heat oven to 200*C & line the baking tray ((swiss roll pan) with butter paper overhanging from the short sides. Cut another sheet of paper larger than the cake pan for rolling the cake later. grease with butter & keep aside.
Using an electric beater whisk egg whites till fluffy, adding sugar gradually. While egg whites are whisking; in a clean mixing bowl combine egg yolks,butter,together with a wire whisk. beat until mixture is creamy
Using a wire whisk, combine egg yolk mixture to the stiffly beaten egg whites,slowly and be careful not to over mix as the whites will deflate causing the roll to be dense. Pour batter onto lined tray and bake in the pre-heated oven for 10 -12 minutes. Remove cake from oven, leave to cool for 5 minutes.
Flip the cake onto the clean piece of greaseproof paper you had cut earlier. Peel the greaseproof paper from the bottom of the cake. Roll cake from the short side into a log and leave to cool completely.
Meanwhile prepare the buttercream . Mix all the ingredients of butter cream except cocoa pwdr . Remove half part of butter cream in a clean bowl .now add the cocoa pwdr to the remaining butter cream & mix well . Refrigerate for few minutes .
Unroll the cake, spread the white butter cream and roll into a log again. Now frost the cake with the cocoa butter cream & keep it in refrigerator for 1/2 hr . for the log effect just make lines on the buttercream with a toothpick or fork . Slice & serve