The Bojon Gourmet

Make the crust:
2 Position a rack in the center of the oven and preheat to 350ºF. Place a 9" tart pan with removable bottom on a rimmed baking sheet.
3 In the bowl of a food processor or stand mixer, combine the almond flour, buckwheat flour, cocoa powder, tapioca starch, muscobado sugar, salt, butter pieces, and vanilla, and process until the mixture forms large, moist clumps, about 30 seconds. (You can also do this in a stand mixer fitted with the paddle attachment or in a bowl with your fingertips.) Dump the crumbly dough into the tart pan. Press the dough first up the sides, then into the bottom of the pan, taking the time to make the crust even and square where the sides meet the bottom. (If the dough is soft or sticky, chill it for 5-10 minutes until it will behave.) Prick the bottom of the dough all over with the tines of a fork.
4 Freeze the crust until firm, at least 15 minutes (or wrap and freeze for up to several months). Place on a rimmed baking sheet and bake until dry, toasty-smelling, and fairly firm to the touch, 20-25 minutes. Scatter the chopped chocolate in the hot shell, let sit for a minute or two to melt, then use the back of a spoon or a small, offset spatula to spread the melted chocolate into a thin layer in the bottom of the tart shell. Let cool to room temperature.
Make the custard filling:
7 In a small saucepan, whisk together the sugar, cornstarch, salt, and vanilla pod and seeds. Whisk in the milk and cream. Bring the mixture to a slow boil over medium-high heat, whisking the dickens out of it, making sure to scrape the bottom and corners of the pans with the whisk. You'll have to stop whisking for a few seconds to verify that the pudding is boiling, which you'll know by the big bubbles that pop gloopily. Once the mixture comes to a boil, continue cooking and whisking for 1 minute. Remove from the heat, whisk in the butter, then the rum. It should be the consistency of creamy yogurt.
8 Strain the pudding through a sieve and into a bowl. Press plastic wrap right on the surface of the pudding, and let cool at room temperature until warm, 30-45 minutes.
Assemble the tart:
11 When the pudding has cooled, spread half of it into the chocolate-lined shell. Peel and slice the bananas, and spread the slices in circles over the pudding. Cover with the remaining pudding. Cover with plastic wrap pressed to the surface of the pudding. Chill until set, at least 2 hours and up to 1 day.
12 Whip the cream with the mascarpone and sugar until it billows softly. Add the rum and vanilla and continue whipping until it holds firm peaks. (If you take the cream too far and it begins to look grainy, you can usually rescue it by gently folding in additional heavy cream until it loosens up again.)
13 Remove the plastic wrap from the tart, and spread the cream evenly over the top, swirling it with the back of a spoon. Sprinkle with the cacao nibs, then use a vegetable peeler to shave chocolate over the top as though you were peeling potatoes (optional).
14 Chill the pie for 30 more minutes to set the cream, if you have the patience. Remove the ring from the pan, place the tart on a cutting board, and use a sharp chef's knife to slice the pie into wedges, wiping the knife clean between each cut.