Grease a half sheet pan and line it with waxed paper or parchment. Preheat the oven to 375.
In a mixing bowl, beat the eggs for 6 minutes on medium. Add the cake mix, oil, and water. Continue mixing for a minute. Scrape the sides of the bowl and continue beating for another minute.
Spread the batter into the prepared pan. Bake for 12 to 14 minutes or until it's springs back in the center.
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. Allow to cool completely.
To make the filling and frosting
While the cake is cooling, whip the cream according to recipe instructions. Store in the fridge.
To make the ganache frosting: In a small saucepan over high heat, bring the cream to a low boil. Remove from heat and whisk in the chocolate chips until smooth. Set aside until it reaches room temperature.
To assemble the buche
Gently unroll cake and spread your chocolate whipped cream 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 room temperature ganache, filling in the gaps between the main log and its branches. Do not frost the cut ends of the cake roll. Scrape the ganache with a fork to resemble bark. Add Playmobil figures or other decorations if you prefer.
Notes
Nutritional values will vary based on what cake and frosting items you choose.