Prepare the filling. Cut the pastry into small 1cm pieces. Arrange on a baking tray, drizzle over melted butter and bake for 30-35 minutes at 170C fan until golden and crisp, tossing halfway through.
Remove half the pastry and set aside. To the remaining, add to a bowl and pour over the pistachio cream and a pinch of salt. Mix well until smooth. You can either freeze this in the bowl itself or create 8-10 small puddles of the mixture on baking paper and freeze.
Make the cookies: Whilst the pastry is in the oven, start the cookie dough. Beat the butter and sugars together until smooth. Then add the vanilla and tahini and beat again to incorporate.
Add in the egg, and beat until completely incorporated, scraping down the sides if needed.
In a separate bowl, whisk together the flour, salt and bicarb, then add this to the cookie batter and mix until just combined – do not overmix.
Finally, add the chocolate chunks, a handful of the excess kataifi and pistachios and mix until dispersed.
Cover the cookie dough and leave to chill in the fridge for 1 hour or overnight.