1. Sift together the flour, cornflour, baking powder, baking soda and salt into a medium bowl and set aside.

2. Place the butter, icing sugar, brown sugar and zest in the bowl of an electric mixer fitted with the paddle attachment and beat on medium speed until lightened, about 2 minutes. Reduce the speed to low and add the egg, then the tahini and vanilla. Beat to incorporate, then add the sifted dry ingredients in 3 batches, again on low speed. Finally, add the chopped chocolate and mix until just combined.

3. Cover the bowl with cling film, pressing so it is in contact with the surface of the dough, and refrigerate for at least an hour, or preferably overnight.

4. When ready to bake, remove the bowl from the fridge and allow to stand at room temperature for about half an hour, or until the dough is soft enough to be scooped.

5. Preheat the oven to 200C (180C fan-forced), and line two oven trays with baking paper. Using a medium-sized ice-cream scoop (about 4.5cm; 30ml capacity) or two spoons, scrape the dough to form balls (roughly 40g) and place on the trays about 5cm apart. Bake for 12-15 minutes, or until the edges of the cookies are golden brown. If baking two trays at the same time, you may need to switch and rotate them so they cook evenly.

6. Remove from oven and allow to cool for a few minutes until cookies are firm enough to handle, then transfer to a wire rack or plate. They are best eaten warm, when the chocolate is still slightly molten, but will keep in an airtight container for 3 days.

Note: The dough, once formed into balls, may also be frozen for up to three months. Bake straight from the freezer, giving them a minute or two longer in the oven.

If you like this recipe, try Helen's spiced chocolate, spelt and hazelnut crackle cookies.