• Preheat your oven to 350°F.
• Grease 15 x 10 inch rimmed baking sheet. Line it with unbleached parchment paper, then grease again.
• Sprinkle some extra flour on the greased paper, and tap it around to evenly distribute it. Set the pan aside.

Make the cake batter.

• In a large bowl, place the flour, xanthan gum, cornstarch, baking powder, baking soda, salt and sugar, and whisk to combine.
• Add the butter, eggs, vanilla and 3 tablespoons water, mixing well after each addition.
• The batter should be thin and easily pourable. If necessary to achieve the proper consistency, add more water by the tablespoon and mix again.
• Pour the batter into the prepared pan, and shake it gently back and forth and side to side so that the batter is in an even layer.
• Smack on the counter to break any trapped bubbles.

Bake the cake.

• Place the pan in the center of the preheated oven and bake for about 15 minutes, or until top of cake springs back when touched.
• While the cake is baking, spread a tea towel out flat on the counter, and sprinkle it liberally with confectioners’ sugar.
• Once the cake is done, turn it immediately onto the prepared towel.
• Carefully peel off the paper, and place a short edge of the towel over the short edge of the cake.
• Roll the cake and towel together tightly, starting with narrow end and rolling away from your body.
• Transfer the rolled cake and towel onto a wire rack to cool completely.

Make the chocolate buttercream filling.

• While the cake is cooling, make the filling. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and shortening until light and fluffy.
• Place the chopped chocolate in a small, microwave-safe bowl. Microwave for 45 seconds at a time on 60% power, stirring well in between microwaving, until melted and smooth.
• Pour the chocolate mixture into the mixer, and beat to combine well.
• Add 2 1/2 cups confectioner’s sugar to the mixer, and mix until well-combined.
• With the mixer on low speed, add the corn syrup.
• The frosting should be smooth but thick enough to hold its shape when scooped with a spoon. If it is too thin, add more confectioners’ sugar by the tablespoon until it reaches the proper consistency.
• Set the filling aside.

Make the chocolate ganache.

• Place the chopped chocolate in a medium-sized heat-safe bowl.
• Place the cream in a small, heavy-bottom saucepan and place over a medium flame until the cream just begins to simmer.
• Pour the cream over the top of the chopped chocolate and allow it to sit undisturbed for a minute or so to melt the chocolate.
• Stir until smooth, and set the ganache aside to thicken as it cools.

Assemble the cake.

• Once the cake is cool, carefully unroll it.
• Remove the towel, and spread the buttercream all over the cake, leaving a 1½ inch clean border all around.
• Carefully reroll the cake tightly.
• Wrap it securely in plastic wrap and place in the freezer until firm, about 15 minutes.
• Remove the cake from the freezer, unwrap the plastic wrap and slice off the very end of both sides of the cake to create clean edges.
• Place the cake, seam side down, on a wire rack above a clean rimmed baking sheet.
• Pour the ganache evenly over the cake, and spread it into an even layer.
• With a small offset spatula or butter knife, create lines along the cake that run along the length of the cake to resemble the bark of a tree.
• If the ganache does not hold its shape in a pattern, place the cake in the refrigerator to cool a bit.
• Place the decorated cake in the refrigerator to set.
• Allow to sit at room temperature for 30 minutes before slicing thickly to serve.