Preheat the oven to 160°C, gas mark 3.
Grease a large skillet.
Cream the butter, tahini and sugar until light and fluffy – about 8 minutes.
Add the egg and vanilla extract and mix well.
In a separate bowl, sift together the flour, bicarbonate of soda and salt, then add the dry ingredients to the creamed butter.
Mix to form a soft dough.
Stir in the chocolate chunks, cover, and allow to rest for 1 hour (or overnight) in the fridge.
Press the cookie dough into a greased skillet and bake for 35-40 minutes or until golden around the edges.
Serve with ice cream.