In a high speed blender, blend the pecans, buckwheat and cacao powder until fine.
Add the prunes and use the tamper to incorporate them into the dry ingredients while blending, until a dough forms.
Remove the dough from the blender and place between two sheets of baking paper and roll to a thickness of about 1cm.
Cut into desired shapes.
Store in an airtight container for up to a few days (if they last that long!), either at room temperature or in the fridge.