Make the pistachio crème: Place the pistachios in a powerful blender and blend until a thick paste forms — at least 5 minutes, so be patient. Add the tahini and continue blending until as smooth as possible, stopping to scrape down the sides (use a tamper if you have one). Once smooth, pour in the melted white chocolate and blend again until the mixture reaches the consistency of Nutella, runny enough to pour into a jar. Prepare the kataifi filling: Melt the butter in a non-stick pan over medium heat. Add the shredded kataifi and fry, stirring constantly, until golden brown and crunchy, a few minutes — keep turning so it browns evenly. Transfer to a bowl and mix in about 80 ml (1/3 cup) of pistachio crème, a little at a time, until you have a thick, paste-like consistency (add more for creamier, less for crunchier). Temper the chocolate: Chop the chocolate into small, even pieces and place in a microwave-safe bowl. Microwave in 30-second intervals, stirring after each, until most has melted; let the residual heat finish melting it as you stir until smooth. Aim for about 88–90°F (31–32°C) for milk chocolate, or 113–115°F (45–46°C) for dark. Assemble the bars: Pour a layer of tempered chocolate into a silicone mould, twisting to coat the sides evenly, then pour off the excess. Place in the fridge or freezer to set. Once set, add the kataifi and pistachio crème filling, pressing it into the corners for an even layer. Pour the remaining chocolate over the filling to seal, spreading evenly. Return to the fridge to set completely before unmoulding