Place the flour, cocoa, baking soda, salt, dark chocolate and granulated sugar in the bowl of a food processor. Blitz together until the chocolate has been broken down into a fine crumb texture. Add the vanilla, butter and coffee and blitz again until the dough comes together. Tip out onto a piece of baking paper, pat out into a square, wrap up in the baking paper and chill for at least 2 hours until firm.