The Bojon Gourmet

Make the cake:
1 Position a rack in the center of the oven and preheat to 350ºLine the bottom of a 9" round cake pan with a round of parchment paper. Cut a strip of parchment paper 5" wide and longer than the circumference of the pan to form a collar on the inside of the pan. This will keep the batter contained as the cake bakes and rises. (Alternatively, you can bake the layers in two 8 or 9" round pans, greased and lined with a round of parchment on the bottom, baking time adjusted.)
2 In a large bowl, sift together the cocoa, chestnut flour, sweet rice flour, tapioca starch, sugar, baking powder, and salt. Make a well in the flour mixture and add the egg yolks, whole eggs, extracts, and oil. Whisk to form a thick, smooth batter, then gradually whisk in the water.
3 In the clean bowl of a stand mixer fitted with the whip attachment, whip the egg whites with the cream of tartar until they hold soft peaks. Gradually sprinkle in the sugar, and continue beating on medium until the whites hold firm peaks and look smooth and glossy.
4 Vigorously stir about one third of the whipped whites into the batter, then gently but quickly fold in the rest of the whites. Pour the batter into the prepared pan. Bake the cake until a toothpick inserted near the center comes out clean, 45-60 minutes. Let the cake cool completely.
5 When the cake is cool, use a small offset spatula or butter knife to loosen the edges and bottom from the pan. Invert it into your hand, and pry off the pan. Remove the parchment and discard. 
6 Place the cake upright on a board, plate, or cake stand (if you have one that rotates, bonus points!). Rub the gooey top layer off the cake gently with your fingertips (this will help the cake absorb the syrup.) With your palm on the top of the cake and the knife held parallel to the work surface, use a sawing motion to cut the cake in half as you rotate it, taking care to make the layers as even as possible.
Make the syrup:
1 Combine the hot coffee with the sugar and stir to dissolve. Stir in the rum.
Make the whipped cream:
1 In the clean bowl of a stand mixer fitted with the whip attachment, combine the chestnut cream and mascarpone, and mash the mascarpone smooth. Add the vanilla bean, heavy cream and sugar, and whip on medium-high until the cream holds soft peaks. Beat in the rum. If the mixture gets over-whipped and starts to look grainy, you can rescue it by gently folding in a few tablespoons of cold heavy cream until it looks smooth again. Refrigerate until needed, up to a few hours.
Assemble the cake:
1 Place the bottom cake round on a circle of parchment paper set on a platter or cake stand. Use a pastry brush to soak the cake with syrup, brushing on a layer, waiting a few minutes to let it absorb, then repeating until the cake is quite moist but not soggy; I used about a third of the syrup here. Top the cake with a little less than half of the whipped cream and spread it almost to the edges.
2 Place the top layer of cake atop the bottom layer, and repeat the syrup-soaking process. Top with the remaining cream, using the back of a spoon to make pretty swoops and swirls. Top the cake with chocolate shavings and cacao nibs. Chill the cake at least an hour to set it, and up to a day or two. You'll want to cover the cake with a cake dome if keeping it for more than a few hours, lest it pick up flavors from the refrigerator.
3 The cake is best within the first two days of making it, but leftovers will keep well, refrigerated airtight, for up to five days.