Start by making the crust. Preheat the oven to 350°F (180°C). Line an 8-inch (20 cm) square pan.
In a medium bowl, use a fork to mix the butter and brown sugar. Add the organic buckwheat flour and organic white buckwheat, then mix again.
Press the mixture into the pan with your fingers. Bake for 20 minutes, or until the crust is lightly browned. Let cool for 15 minutes after removing from the oven.
For the chocolate ganache. While the crust is cooling, heat the milk and cream in a saucepan. Chop the dark chocolate and place it in a bowl. Pour the hot cream and milk mixture over the dark chocolate. Let stand for 5 minutes. Mix well with a spatula until the mixture is smooth and glossy.
Pour the ganache over the cooled crust. Refrigerate for at least two hours before serving.
