Step #1 THE FILLING & FROSTING Beat the smoothly melted chocolate into the meringue, along with the vanilla, rum, & soft butter.
Step #2 Remove two-thirds of the mixture to a bowl & put in the fridge; this is the frosting.
Step #3 Beat the extra half stick of soft butter into the remaining meringue this is the filling.
Step #4 FILLING THE CAKE Unroll the sponge sheet & slice a quarter inch off the long sides so that the cake will roll easily.
Step #5 Spread the filling over the top of the sheet.
Step #6 Roll it up from one of the small ends & you have made the log shape.
Step #7 Neatly slice a narrow slanting piece from each end of the log.
Step #8 With a small knife dig a hole in an upper side of the log & insert a piece of the scrap to simulate a bump or a branch.
Step #9 Place the log seam-side down on the serving board & slip double sheets of wax paper under the edge of each side & the two ends-to catch frosting spills.
Step #10 FROSTING THE LOG Beat 2 or more spoonfuls of sifted cocoa into the frosting mixture to make it a spreadable consistency.
Step #11 (Reserve 2 spoonfuls of frosting for the mushrooms.
Step #12 ) Leaving the two ends free, frost the cake using a flexible metal spatula, then scumble it with the spatula & a fork to give it a rough barklike look.
Step #13 Ahead-of-time note: put in the fridge 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.
Step #14 FINAL DECORATIONS, JUST BEFORE SERVING The mushrooms.
Step #15 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 & then the pointed end of a meringue stem.
Step #16 Arrange the mushrooms in tasteful clusters upon the log & Sprinkle top the log with a dusting of confectioner's sugar to give a snowy effect.
Step #17 Dust the tops of the mushrooms with cocoa powder tapped from the sieve.
Step #18 The caramel veil.
Step #19 Bring the sugar to the simmer with the corn syrup & one-third c water in a small saucepan; when the mixture is completely dissolved & clear, cover tightly & boil to the caramel stage.
Step #20 Let it cool two to three mins, until it forms thick strands when lifted with a fork.
Step #21 Then dip the fork into the syrup & wave it over the broomstick, to form long hanging threads of caramel.
Step #22 The finale.
Step #23 Lift the caramel strands off the broomstick; drape them over the yule log.
Step #24 Decorate the serving board with sprigs of holly if you wish, & the buche de Noel is ready to serve.