First, prepare the crust. Set a pan over medium-high heat. Add the buckwheat flour and cook for about 5 - 10 minutes, or until the flour starts to brown and a nutty aroma forms. Remove from the heat and sift the flour into a bowl to cool. Add the cacao powder and softened butter. Lightly rub the butter into the flour using your fingertips until there are no more big lumps of butter. Add sugar, nutmeg, cloves, and 1 tsp of lemon zest. Stir well, add eggs and one pinch of salt. Combine all ingredients with your hands until the dough comes together but remains crumbly. Knead for about 30 seconds. Using your hands, form a ball of dough, wrap it in clingfilm, and place it in the fridge for 1 hour.
Bake the crust
Preheat the oven to 175 °C / 350 °F. Lightly dust a clean work surface with flour, flatten the dough into a disk and roll out the tart dough to 0.5 cm / 0.2 inch thick. Transfer the dough into a round tart pan (24cm, 4 cm high / ⌀ 9-inch, 1.5 inch high). Prick the dough all over with the tines of a fork. Press the tines up against the edges of the tart shell—Line the dough with a large piece of parchment paper. Press the paper into the edges of the crust too. Fill the parchment paper with pie weights or dried beans. Make sure they cover the entire bottom of the tart dough. Bake partially. First, bake for 20 minutes, then check if the parchment paper sticks. If not, remove the paper with the beans or pie weight and bake for another 15 minutes. Let the tart crust cool on a wire rack.
While the crust is cooling, prepare the caramel. Add sugar to a saucepan and place over high heat for the sugar to dissolve and caramel form. Remove from the heat. Gradually add in the mascarpone, spoon by spoon, constantly stirring, fast. Be careful not to create lumps. Pour the mixture into the baked buckwheat crust.TipBefore adding mascarpone to the caramel, make sure that the mascarpone is at room temperature to reduce the risk of lumps. If, for some reason, lumps start to appear in the caramel, don't worry too much; place it back on the heat, stirring constantly, and you will get rid of it.
In a large bowl, mix chestnut puree, rum, and mascarpone. In a small separate bowl, combine the butter and cooked, peeled chestnuts (you can either mix well to get a smooth, creamy texture or leave some bits of chestnuts, depending on your taste - we like both textures). Add to the chestnut puree mixture and combine well, using an electric mixer. Add in the melted chocolate and mix well. Using a spatula, spread the chestnut pie filling evenly all over the cooled caramel in the tart
soft chocolate ganache
Place the whipping cream in a small saucepan and bring to a boil. Melt half of the chocolate, then add the rest of the chocolate, stir to combine. Pour hot cream over the chocolate. Mix well, then sour over the chestnut pie filling. Let the tart rest for 30 minutes in a refrigerator, then serve.