Place chocolate chips in the fridge or freezer to prevent them from melting during baking.
Preheat the oven to 180°C and line baking sheets with parchment paper.
Roughly chop the dark chocolate and melt it in the microwave or over simmering water.
Whisk together the buckwheat flour, cocoa, bicarbonate of soda, and salt in a bowl.
Cream the butter, sugar, and vanilla until fluffy.
Beat in the melted chocolate followed by the cold eggs one at a time.
Mix in the dry ingredients at a low speed until combined.
Fold in the chilled chocolate chips with a spatula.
Dollop rounded tablespoons of dough onto the sheets, spaced 6cm apart.
Bake for 9 to 10 minutes until edges are set but centers appear soft.
Rest cookies on the tray for 10 minutes before moving to a wire rack.