Grate or shred the chocolate and melt it slowly with water without allowing it to boil. Cream butter and sugar. Add 1 egg and a quarter of the flour, and beat until well mixed. Add remaining egg and then the rest of the flour, beating until all is well mixed. The lightness of the roll depends upon the mixture being well beaten. Flavour with cooled melted chocolate and vanilla extract, to taste. Stir in baking powder and mix well.
Line a rectangular baking tin with waxed paper. Butter the paper well and spread the batter evenly over it. (The mixture should not be more than ¼ inch thick, or it will not roll well when baked.)
Turn the cake out on to a board dusted with sugar; remove the paper from the cake; sprinkle more sugar over the top of the cake; trim the edges and roll up quickly. When cold, unroll; spread with Vanilla Butter Icing and then re-roll the cake. Ice with Chocolate or Vanilla Butter Icing if desired, or dust with powdered sugar.