Make the Meringue Mushrooms, Chocolate Buttercream, Almond Pastry Cream and the Basic Chocolate Genoise.
To assemble the cake, carefully unroll the genoise. Spread the pastry cream evenly over the cake leaving about 1 to 2 inches on the long side plain. Reroll the cake, wrap with plastic wrap and chill until firm, about 1 hour.
Place the cake seam side down on a platter and tuck parchement paper around it to keep the platter clean while you ice the cake.
Cut two wedges off the end of each side of the cake at 45 degree angles and set aside. Ice the log with a thin layer of buttercream. Attach the wedges on a diagonal on opposite side of a cake to form what will look like a tree limb.
Spread the remaining buttercream over the cake. Use a small, offset spatula to create a bark-like texture. Chill the cake for at least 30 minutes and up to 1 day before finishing decorating.
When ready to serve, arrange the mushrooms around the cake, sprinkle the shaved chocolate over the cake and platter and dust lightly with confectioners' sugar. Use fresh holly leaves or other greenery to decorate the platter.