Clatter the chocolate chips out into a flattish dish and put this in the fridge while you get on with making the batter. It wouldn’t hurt to sit them in the freezer, either. I do this so that the chips don’t melt too much while baking, leaving you with nuggets of intense chocolatiness.
Preheat the oven to 180C/160C Fan/Gas 4. Line a couple of baking sheets (or 1 if baking in 2 batches) with baking parchment.
Roughly chop the dark chocolate, and melt it either in a suitable bowl in the microwave or over a pan of simmering water. Set aside to cool a little.
In another bowl, mix together the buckwheat flour, cocoa, bicarb and salt, and fork to make sure everything’s well combined.
In yet another bowl (I use my freestanding mixer here, but a bowl and a handheld whisk or, indeed, wooden spoon and elbow grease would work, too), cream together the butter and sugar with the vanilla extract until a dark caramel colour and fluffy, using a spatula to scrape down the sides of the bowl if necessary. Beat in the cooled, melted chocolate then the fridge-cold eggs (I find this means one doesn’t then have to refrigerate the dough before baking) one by one, and when both are absorbed into the butter and sugar mixture, scrape down the sides of the bowl again, turn the speed down and carefully beat in the dry ingredients.
Using a wooden spoon or a spatula, fold in the cold chocolate chips, then dollop rounded tablespoonsful of the dough onto a lined baking sheet, leaving about 6cm/2½in between each one. Put the bowl with the remaining cookie dough in the fridge while the first batch is underway.
Bake for 9–10 minutes, by which time the cookies will be just set at the edges, but otherwise seem undercooked, then remove the sheet from the oven and let the cookies sit on the warm tray for another 10 minutes before transferring them to a wire rack to cool.
When the tray is cool, or you have another one lined and ready to go, take the bowl of dough out of the fridge and proceed as before.