Make chocolate genoise and mousse, ganache icing, and meringue mushrooms.
To assemble cake, carefully unroll genoise on the back side of a baking sheet (discard the plastic wrap and waxed paper, but keep the towel). Spread chocolate mousse evenly on cake to within 1 to 2 inches of one long end. Reroll cake, starting from other long end, using towel to help roll it. Cover with plastic wrap; chill until firm, about 1 hour.
Place cake, seam side down, on a serving platter; tuck parchment around it to keep platter clean while decorating.
Whip ganache at medium speed until it has the consistency of soft butter. Cut two wedges off ends of cake at a 45 degrees angle; set aside. Ice log with a thin layer of ganache. Attach wedges on diagonally opposite sides of log. Spread ganache all over log, using a small spatula to form barklike ridges. Chill until ganache is firm, about 30 minutes.
In the top of a double boiler or in a heat-proof bowl set over simmering water, melt chocolate until smooth. Line a baking sheet with parchment. Spread melted chocolate 1/8 inch thick over parchment. Refrigerate until cold, 10 to 15 minutes. Roll paper back and forth until chocolate splinters; sprinkle over cake. Chill cake until ready to serve.
When ready to serve, arrange meringue mushrooms around and on cake, and dust lightly with confectioners' sugar.