Start by roughly chopping the chocolate into shards of the kind of size you’d like to find in a biscuit.
Put the butter in a food mixer and beat briefly to soften, then add the sugar and syrup, and beat on a medium speed for about five minutes, scraping down the bowl as necessary, until fluffy.
Meanwhile, sift together the flour, cocoa powder and salt. With the mixer running on a low speed, add the dry ingredients and continue mixing until everything comes together into a dough. Which it will, don’t worry.
Roll into a sausage shape about 5cm in diameter. Wrap well and chill for at least 30 minutes, and up to 10 days.
Heat the oven to 180C (160C fan)/350F/gas 4 and line two baking trays with greaseproof paper. Cut the sausage into thin rounds and arrange, fairly well spaced out, on the trays. Bake for 25 minutes (for crisp biscuits; or 15 for squidgy, or 20 for somewhere in between), then leave to cool on the baking tray.