** The Filling And Frosting **
Beat the smoothly melted chocolate into the meringue, along with the vanilla, rum, and soft butter. Remove two-thirds of the mixture to a bowl and refrigerate; this is the frosting. Beat the additional half stick of soft butter into the remaining meringue this is the filling.
** Filling The Cake **
Unroll the sponge sheet and slice a quarter inch off the long sides so that the cake will roll easily. Spread the filling over the top of the sheet. Roll it up from one of the small ends and you have made the log shape. Neatly slice a narrow slanting piece from each end of the log. With a small knife dig a hole in an upper side of the log and insert a piece of the scrap to simulate a bump or a branch. Place the log seam-side down on the serving board and slip double sheets of wax paper under the edge of each side and the two ends-to catch frosting spills.
** Frosting The Log **
Beat 2 or more spoonfuls of sifted cocoa into the frosting mixture to make it a spreadable consistency. (Reserve 2 spoonfuls of frosting for the mushrooms.) Leaving the two ends free, frost the cake using a flexible metal spatula, then scumble it with the spatula and a fork to give it a rough barklike look.
** Ahead-of-time note: Refrigerate the log if you are not contin- uing; it will keep nicely for a day-if you can find a non-mashing cover for it.
** Final Decorations, Just Before Serving **
The mushrooms. With a small knife, pierce a hole in the bottom of each meringue mushroom cap, insert a bit of the frosting (or softened butter) into the hole and then the pointed end of a meringue stem. Arrange the mushrooms in tasteful clusters upon the log and sprinkle the log with a dusting of confectioner's sugar to give a snowy effect. Dust the tops of the mushrooms with cocoa powder tapped from the sieve.
The caramel veil. Bring the sugar to the simmer with the corn syrup and one-third cup water in a small saucepan; when the mixture is completely dissolved and clear, cover tightly and boil to the caramel stage. Let it cool two to three minutes, until it forms thick strands when lifted with a fork. Then dip the fork into the syrup and wave it over the broomstick, to form long hanging threads of caramel.
The finale. Lift the caramel strands off the broomstick; drape them over the yule log. Decorate the serving board with sprigs of holly if you wish, and the buche de Noel is ready to serve.