Grease a 15x10x1-inch jelly-roll pan. Line pan with waxed paper. Generously grease waxed paper. Set aside.
In medium mixing bowl, combine cake flour, cornstarch, cocoa powder, soda, and salt. In large mixing bowl, beat eggs, yolks, and the 1 teaspoon vanilla with electric mixer on high speed 4 to 5 minutes or until thick and lemon colored. Gradually add 3/4 cup sugar, beating 2 minutes on high to dissolve sugar.
Sprinkle flour mixture, one third at a time, over egg mixture and use a large rubber spatula to gently fold until mixed.
Evenly spread batter in the prepared jelly-roll pan. Bake in a 375 degree F oven about 12 minutes or until the cake springs back when lightly touched with your finger.
Remove the cake from the oven. Cool in pan on a wire rack for 3 minutes. Generously sprinkle the powdered sugar over a clean kitchen towel thats slightly larger than the cake. Invert pan and turn cake upside down onto the towel. Carefully remove waxed paper by peeling it off the cake; discard.
While cake is warm, roll up cake and towel, starting from one of the cakes short sides.* (Its important to roll the cake while warm to avoid cracking.) Cool rolled cake on a wire rack.
Gently unroll cake; remove towel. In a chilled medium mixing bowl, combine whipping cream, the 1/4 cup powdered sugar, and the 1/2 teaspoon vanilla. eat with chilled beaters of an electric mixer on medium speed until soft peaks form. With a thin metal spatula, spread the whipped cream filling on the cake to within 1 inch of the edges.
Carefully roll up cake (without the towel) starting from a short side.* Place on serving platter, seam side down, and chill in the refrigerator for 30 minutes.
Remove cake from refrigerator. With a serrated knife, diagonally cut off a 3-inch slice from one end of cake. Place the diagonally cut edge of piece against side of longer roll on the serving plate. This forms a branch on the log.
Frost entire cake with So-Chocolate Buttercream, carefully cementing limb to trunk with buttercream. Chill again until buttercream firms, if necessary. With the tines of a table fork, create a pattern of rings on the cut ends of the roll, and bark on the rest of the tree. Place the candied flowers and leaves on the bark in a random pattern, if you like. Using a sieve, gently dust the cake with 2 tablespoons powdered sugar to create a sprinkling of snow. Chill cake until ready to serve.
*For a longer cake: Roll cake from a long side for a longer, thinner log.
For a longer cake: Roll cake from a long side for a longer, thinner log.