In a medium bowl combine the flour, baking soda and baking powder, and salt.
In a small bowl, stir the cornflour and milk together. Add it to a mixing bowl withg the melted (and cooled) melted butter, brown and granulated sugar, tahini and vanilla.
Add the wet ingredients to the dry and mix together until almost combined. Stir in the chocolate chunks and stir until combined.
Cover the bowl with cling film and place the dough in the refrigerator for at least 30 minutes and up to 24 hours. This will firm up the dough making it easier to handle, enhance the flavours and create a chewier baked cookie.
Preheat the oven to 180° C (356° F) and line a baking tray with baking paper.
Using an ice-cream scoop or your hands, roll a heaped tablespoon of the refrigerated dough in to a rough ball and lay 6 to 8 balls on your baking tray (depending on its size) and pop in the oven for 13 minutes - 16 minutes.
Remove the baked cookies from the oven and allow them to cool for 15-minutes (they will deflate and crack beautifully during this time) before gently setting them aside to continue baking the remaining dough.
Roll and bake the remaining dough. Store in an airtight container for up to 4 days.