Make meringue mushrooms up to 3 days ahead. Preheat the oven to 225º. Line the baking sheet with parchment and set aside.
Use an electric mixer to whip egg whites till foamy. Add cream of tartar, salt, and vanilla. Continue whipping till egg whites form soft peaks.
Slowly add superfine sugar while continuing to whip on high till the whites form stiff peaks.
Place the meringue in a pastry bag fitted with a large plain tip. For stems, press out a bit of meringue onto the baking sheet and lift the tip straight up.
For caps, squeeze out round mounds of meringue. Bake for 1 hour. Remove from oven and cool completely.
To make mushrooms, melt semi-sweet chocolate in the microwave. Add vegetable oil and stir. Use a toothpick to make a hole in the bottom of the cap.
Spread a small amount of chocolate on the underside of the cap and connect the stem to the cap. Dust mushrooms with cocoa powder to finish.
Make chocolate buttercream by melting chocolates and beating with butter, powdered sugar, cocoa powder, milk, and vanilla.
Preheat oven to 350º. Butter 10½ x 15½ x 1-inch jelly roll pan. Line with parchment paper then butter and flour paper.
Sift flour, cocoa, and baking soda twice into a medium bowl. Set aside while you clarify the butter.
In a medium heat-proof bowl, whisk together sugar and eggs. Heat till the mixture is warm and sugar is dissolved. Beat on high till tripled in volume.
Sift about a third of the dry ingredients into the egg mixture. Fold in with a spatula. Repeat two more times, adding in the butter with the last addition.
Spread batter evenly in the prepared jelly roll pan. Bake for 15-20 minutes, till cake pops back up when lightly tapped. Place on rack till cool enough to handle.
Dust the top of the cake with cocoa powder. Cut around the perimeter of the cake to loosen. Invert the cake onto a work surface, peel off the parchment, sprinkle with cocoa.
Starting with one short end, carefully roll up the cake in the towel. Wrap in plastic if desired and refrigerate till needed.
Make ganache by microwaving cream till hot. Add chocolate and whisk till smooth. Refrigerate till cold.
Whip cold ganache at medium speed till it has the consistency of soft butter. Start assembling the cake.
Carefully unroll the cake onto the backside of a baking pan. Spread mousse on cake leaving a one-inch margin on each long edge.
Reroll cake starting with the other end. Cover with plastic and chill for one hour.
Ice with the ganache. Make lines or ridges on the cake to resemble tree bark if desired. Garnish with meringue mushrooms.
