Grease a 200 mm. X 150 mm. (10” x 6”) aluminium tin with butter, place a butter paper of the same size over it and again grease the butter paper with butter. Keep aside. Combine the eggs and castor sugar in a deep bowl, mix well and beat using an electric beater for 3 to 4 minutes on a medium speed, till it is light and fluffy. Add the plain flour, cocoa powder and vanilla essence and mix gently with the help of a spatula. Transfer the batter into the greased tin and bake in an pre-heated at 200°c (400°f) for 8 minutes. Keep aside to cool slightly. Once it is slightly cooled, loosen the sides of the cake using a sharp knife. Sprinkle the powdered sugar evenly on a clean, dry surface and invert the cake tin over it. Slowly peel the butter paper and discard it. Spread the whipped cream evenly over the cake and roll it gently from one side to the other to form a swiss roll. Place the roll on a serving plate and apply the chocolate sauce evenly over it with the help of a palette knife. Cut the roll into 7 equal portions using a sharp knife. Serve immediately.