Sift together the plain flour, cornflour and cocoa powder.
In a separate bowl, whisk together the butter and sugar with an electric mixer until light and fluffy.
Whisk in the orange zest, vanilla extract and a pinch of salt.
Add the dry ingredients and stir in until all the flour is moistened and no lumps of butter remain.
Stir through the chopped chocolate then use your hands to bring the dough together.
Form the dough into a log about 5-7cm (2-2 ¾ in) thick - use your hands and roll it on a surface to get it as even as possible.
Wrap the log in baking parchment and refrigerate for 2 hours, and up to 3 days.
Use a very sharp knife to slice the log into 1cm (⅓ in) thick slices. They may crumble a bit due to the chunks of chocolate, if this happens just use your hands to press the dough back into a round. (If the dough has become too firm to slice, let it sit out at room temperature for about half an hour before slicing it).
Place the cookie slices slightly spaced apart on a couple of parchment lined baking sheets (they will spread a little in the oven) and pop them in the freezer for 15 minutes while you preheat the oven to 180°C/160°C fan/350°F/gas mark 4.
Bake the cookies for about 20 minutes. They should feel dry to the touch and be set around the edges. It is difficult to judge when they are ready so you may wish to bake a test one first to judge how long they need in your oven.
Allow to cool on the tray for 5 minutes then carefully transfer them to a wire rack to cool completely. Once cold store in an airtight container.