Line a jelly roll pan with parchment paper. If using an elaborate pattern, create a template and place it beneath the parchment.
Beat egg white in a small bowl. Add sugar and flour, then mix well. Beat in butter. Batter will be paste-like. Divide if necessary to add color as desired. Put batter in piping bags, then pipe the desired pattern onto the parchment paper. Put in the freezer for 15 minutes, and prepare the cake batter.
Cake
Preheat oven to 375.
Beat the eggs for 5 minutes. Gradually add granulated sugar and beat for an additional 5 minutes until the mixture is pale. Add vanilla.
In a separate bowl, sift together the flour, cocoa powder, and baking powder. Fold into the egg mixture taking care to not overmix, then add water and coloring. Stir until combined.
Remove the jelly roll pan from the freezer, and slide out the pattern sheet if necessary. Pour the batter over the piped pattern. Bake for 10 minutes or until springs back when pressed gently.
Filling
Combine whipping cream, powdered sugar, and vanilla. Whip until stiff peaks form.
Assembly
Flip cake out onto a piece of parchment or wax paper. Once flipped, pull the parchment paper off of the decorated side. Flip it over again, this time onto a clean dish towel. Roll the cake, starting from the side opposite from the pattern, and allow to cool.
Unroll the cake and spread it with icing. Roll cake again into a tight spiral. Slice off the ends of the cake with a serrated knife to clean up the edges if necessary.