First prepare the crust. Set a pan over medium high heat. Add the buckwheat flour and cook for about 5 - 10 minutes, so that the flour gets a bit brown and smells wonderfully nutty. Remove from the heat and sift the flour into a medium bowl to cool down. 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 sea salt. Combine all ingredients with your hands, until the dough starts to come together, but it is still crumbly. Knead for about 30 seconds. Using your hands, form a ball of dough, wrap in plastic wrap and set to cool in a refrigerator 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. Line tart shell with the rolled out dough, using a rolling pin. Prick the dough all over with the tines of a fork, and 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 dough. Fill the parchment paper lined tart dough 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, then 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 dry pan. Heat over high heat to dissolve the sugar until it becomes dark caramel. Remove from the heat. Gradually add the mascarpone, spoon by spoon, stirring constantly, 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 together chestnut puree, rum and mascarpone. In a small separate bowl mix together the butter and cooked, peeled chestnuts (you can either mix well to get a nice, creamy texture or leave some bits of chestnuts, depending on your taste - we love both). Add to the chestnut puree mixture and combine well, using a hand mixer. Melt the chocolatein a glass bowl over a pan of simmering water, stiring occasionally. When the chocolate melts completely slowly add it to the chestnut puree mixture and combine well, using a hand mixer. Spread the chestnut pie filling evenly all over the cooled caramel, using a spatula.
soft chocolate ganache
Place the double cream in a small saucepan and bring to a boil. Melt 75g / 2.6 oz of the chocolate in a glass bowl over a pan of simmering water, stiring occasionally. Add the rest of the chocolate. Pour hot double cream over the chocolate. Using a hand blender mix all together. Pour over the chestnut pie filling evenly. Let the pie rest for 30 minutes in a refrigerator, then serve. Bon appetite.