Unroll the cake and spread all but a 1/4 cup of the cooled ganache over the complete surface of the cake with an offset spatula.
Using the parchment to help you roll the cake over the ganache into a tight log starting at the long end.
Wrap the log of cake and refrigerate for at least an hour to set the ganache, which will make the cake easier to work with.
Once the cake is chilled cut it at an angle in half. Cut one of the halves again making one of the pieces 1/4 and the other 3/4. Place the 3/4 piece against the half log and the 1/4 piece will sit on top of the half log.
Spread the buttercream over the surface of the cake. You want to create the appearance of bark so don’t worry about it being too smooth, you probably will not use all of the buttercream. I even swirl the ends to resemble the cut end of the log. Save at least a small portion of the buttercream to mix with the reserved ganache.
Blend the reserved ganache with the buttercream to create a contrasting color. Use your spatula to spread this onto the log in patches. You want to be able to see the lighter buttercream underneath.
Use a fork to create the bark texture on the Bûche de Noël and swirl the two buttercream colors together.