Cook the buckwheat flour
Set a skillet over medium high heat. Add the buckwheat flour and cook it for about 5 - 10 minutes, stirring occasionally. It will start to smell nutty and wonderful. Remove from the heat and sift the flour and cacao powder into a bowl to cool down. Rub the softened butter into the dry ingredients using your fingertips until there are no more big lumps of butter.
Knead into a dough
Add in the sugar, ground nutmeg, ground cloves, 1 tsp of lemon zest, eggs and a pinch of salt. Gently knead it together using your hands until you have a ball of dough. The dough still needs to be chewy and crumbly. Wrap in a clingfilm and place into the fridge for 1 hour.
roll out the dough
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.Carefully roll your tart dough around the rolling pin, then unroll it carefully over your tart pan. Prick the dough all over with the tines of a fork, and press the tines up against the edges of the tart crust. Line the dough with a large piece of parchment paper. Press the paper into the edges of the dough. Fill it with pie weights or dried beans. Make sure they cover the entire bottom of the tart crust.
Bake for 20 minutes, then check if the parchment paper still sticks to the tart crust. 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 then fill it with your favorite tart filling. Enjoy.