In a bain-marie or microwave, melt the butter and unsweetened chocolate over very low heat, stirring constantly, in 30-second pulses, removing the chocolate when it is almost, but not completely, melted. Remove from the heat and stir until the mixture is homogeneous.
Add the sugar to the melted butter and chocolate, then add the eggs one at a time and the vanilla. Whisk in the baking soda and salt. If the cocoa powder is lumpy, sift it directly onto the surface of the batter; otherwise, simply add it to the bowl. Add the buckwheat flour and stir until everything is well mixed. Add the chocolate chips and stir until everything is well mixed.
Place the bowl in the refrigerator for about 30 minutes, or up to a few days. In less than 30 minutes, you can bake the dough right away; it will hold its shape better and be easier to pick up when cooled. After more than 30 minutes, the dough will become difficult to shape.
Preheat the oven to 350°F (180°C).
Place about 1 1/2 to 2 tablespoons (25 to 30 ml) of cookie dough on a baking sheet lined with parchment paper or a nonstick spray, leaving enough space for the cookies to spread a little. Bake for 11 to 12 minutes, for cookies with a soft center; the cookies will look undercooked. Let the cookies rest on the baking sheets for a few minutes before carefully transferring them to a wire rack to cool before eating.
The cookie dough can be stored in the refrigerator for up to a week, and longer in the freezer.
