Grease a jelly roll pan and line it with waxed paper or parchment. Bake the cake in the prepared jelly roll pan according to package directions.
Meanwhile, sift cocoa or powdered sugar over a clean tea towel. Invert warm cake onto towel. Remove parchment paper. Roll cake up in the towel. Yes, really. You want it to cool with a curve to it otherwise the cake will break.
Gently unroll cake and spread your filling. Reroll, minus the towel. Wrap in plastic wrap and chill until firm, about 1 hour.
Remove plastic wrap and cut off 2-3 inches off each end, at an angle. Place large piece of cake, seam side down, on a serving platter. Position smaller pieces on the side of the main log. These are like "branches" to the cake.
Frost cake with chocolate frosting, filling in the gaps between the main log and its branches. Do not frost the ends. Scrape frosting with a fork to resemble bark. Add Playmobil figures or other decorations if you prefer.