Prep
Line a loaf pan or 8×8-inch pan with parchment paper, leaving overhang.
Finely chop the chocolate and set aside.
Loosen kataifi strands so they toast evenly.
Toast the Kataifi
Melt butter in a large skillet over medium heat.
Add kataifi and cook, stirring constantly, for 8–10 minutes until golden and crisp.
Transfer to a plate and let cool completely (crucial for crunch).
Make the Pistachio Filling
In a bowl, mix pistachio paste, powdered sugar (if using), tahini, vanilla, and salt until smooth and spreadable. Set aside.
Melt & Layer the Chocolate
Gently melt ⅔ of the chocolate over a double boiler or in short microwave bursts.
Remove from heat and stir in the remaining ⅓ chocolate until smooth (easy tempering).
Stir in oil or cocoa butter if using.
Pour half the melted chocolate into the lined pan and spread evenly. Chill 10 minutes until just set.
Spread pistachio filling evenly over the chocolate.
Sprinkle cooled kataifi on top and press gently.
Pour remaining chocolate over the top and smooth.
Set & Slice
Chill for 30–45 minutes until fully firm.
Lift out using parchment and slice with a warm knife for clean edges.