There are some recipes, such as the "Bûche de Noël" -- or yule-log cake, where a tree-bark-like texture adds to the appearance of the cake. The tree-bark look can be simple or complex, from a wood-grain pattern to stripes that suggest bark. Purchase plain fondant or make it out of marshmallow cream and confectioners' sugar; look for recipes online. You can find textured mats in craft shops or online.
Melt cocoa discs in the microwave in 30-second intervals, until they are melted. Sir in corn syrup. Pour the mixture on a sheet of wax paper and cool it completely.
Knead a piece of the cocoa mixture until it's soft. Gradually knead the cocoa mixture into the fondant until it's a uniform brown color. If the color is too light, knead in some black-gel food coloring.
Dust a sheet of wax paper with cocoa powder. Place the fondant on top and dust it with more cocoa powder. Put a second piece of wax paper on top and roll out the fondant with a rolling pin until it's 1/8- to 1/4-inch thick.
Remove the top piece of wax paper. Use a sharp knife to "draw" a bark texture on the wood by pulling the knife vertically across the fondant. The lines should occasionally intersect, and circular "knots" can be drawn in with the knife. Alternately, lay a wood-grain mat over the fondant and press hard; remove the mat slowly to reveal the texture.
Roll additional chocolate fondant into thin ropes and press them gently over the lines to add three-dimensional tree-bark texture.
Use a texturing scraper or cake comb, found at most craft shops in the clay or cake decorating department, to make several parallel lines at once. Twist while scraping to give it a simple tree-bark appearance.