Preheat the oven to 338°F/170°C.
In a bowl, combine the dry ingredients - buckwheat flour, cocoa powder, ground hazelnuts, cinnamon, salt, and a small pinch of baking powder.
In a separate bowl, beat the butter with powdered sugar until smooth and slightly airy. Add the egg and mix well until everything is nicely combined.
Gradually add the dry mixture to the butter mixture. At first, the dough will look crumbly and coarse. Keep mixing until it comes together into a soft dough and turns a rich brown color. You can use a mixer or knead it by hand; the dough should be easy to handle and not overly sticky.
Shape the dough into a ball, cover it, and refrigerate for about 30 minutes. This makes it firmer and easier to shape later.
Line a baking sheet with parchment paper. Portion the dough into tablespoon-sized pieces and roll them into balls. Keep them even in size and place them on the tray with about 4 cm (1½ inches) of space between each.
Press an indentation into each ball using your thumb or the back of a teaspoon. Lightly greasing the spoon helps keep the shape clean.
Bake for 10 minutes. Since the cookies are dark, check them closely so they don’t overbake.
While the cookies are in the oven, prepare the filling. Warm the heavy cream and melt the chocolate in it, then stir in the orange zest. For a shinier finish, you can add a tablespoon of coconut oil.
Take the cookies out of the oven and fill the centers right away while they’re still warm. A teaspoon works perfectly, or use a piping bag if you prefer a neater look. Handle them gently as they are very soft when hot. If the filling thickens, warm it slightly again. Transfer the cookies to a cooling rack once they're done.
Let them cool completely so the filling sets. Once firm, they can be stacked without sticking.