Start with the crust. Preheat the oven to 350 ° F (180 ° C). Line a 20 cm (8 inches) round pan.
In a medium bowl, using a fork, combine the butter and the light sugar. Add organic buckwheat flour and organic white buckwheat, then mix again.
Put the preparation in the mould by pressing it well to the bottom with your fingers. Bake for 20 minutes or until the crust is lightly golden. Let cool for 15 minutes out of the oven.
For the chocolate ganache. While the crust is warming. In a saucepan, heat the milk and cream. Chop the dark chocolate and put it in a bowl. Pour the cream and hot milk mixture over the dark chocolate. Let stand 5 minutes. Mix well with a spatula until the mixture becomes smooth and shiny.
Pour the ganache on the crust and refrigerate the tart for at least 2 hours.