Adapted from Martha Stewart
5 tablespoons butter, plus more to butter pan
2/3 cup cake flour
1/3 cup cocoa powder, plus more for dusting cake
Pinch of baking soda
3/4 cup sugar
2 teaspoons vanilla
Preheat oven to 350º. Butter 10 1/2 x 15 1/2 x 1 inch jelly roll pan. Line with parchment paper then butter and flour paper. Tap out excess flour.
Sift flour, cocoa and baking soda twice into a medium bowl. Set aside while you clarify the butter. Over low heat, melt butter. Skim off white foam and then pour butter into a bowl, leaving any white solids which remain in the pan. Set aside and don't forget to add this to the cake batter (I nearly did).
In a medium heat-proof bowl, whisk together sugar and eggs. Place over a pot of simmering water and heat and whisk till mixture is warm and sugar is dissolved. Remove from heat and beat on high till tripled in volume. Reduce speed to medium and add vanilla. Beat another few minutes.
Sift about a third of the dry ingredients into egg mixture. Fold in with spatula. Repeat two more times, adding in the butter with the last addition.
Spread batter evenly in prepared jelly roll pan. Bake 15-20 minutes, till cake pops back up when lightly tapped. Do not overbake. Place on rack till cool enough to handle.
Dust top of cake with cocoa powder. Cut around perimeter of cake to loosen. Place a piece of waxed paper over top of cake, followed by a damp dish towel. Invert cake onto a work surface, peel off parchment and sprinkle with cocoa. Starting with one short end, carefully roll up cake in the towel. Wrap in plastic if desired and refrigerate till needed.
4 ounces semi-sweet chocolate, chopped
4 tablespoons butter, cut into 4 pieces
4 eggs separated
Pinch of cream of tartar
1/2 cup heavy cream
Melt chocolate and butter in microwave, stopping and stirring at 30 second increments till smooth. Let cool slightly. Whisk in egg yolks. Let cool to room temperature.
Whip the heavy cream till soft peaks form, and fold into the chocolate mixture. Chill till set, about an hour.
Chocolate Ganache Icing~
6 ounces semi-sweet chocolate, chopped
1 cup heavy cream
Microwave cream in large measuring cup till hot. Add chocolate and whisk till smooth. Refrigerated till cold.
Whip cold ganache at medium speed till it has the consistency of soft butter. Start assembling the cake.
Carefully unroll cake onto the backside of a baking pan. Discard the wax paper and any plastic wrap. Spread mousse on cake leaving a one inch margin on each long edge. Reroll cake starting with the other end...using towel to help roll. Cover with plastic and chill one hour.
Ice with the ganache. Make lines or ridges on cake to resemble tree bark if desired. Before icing you can also cut off two wedges at 45º angles and attach with icing on diagonally opposite sides of the log. I skipped this step.
After slicing, sprinkle with powdered sugar and garnish with meringue mushrooms.