Make sure molds, bowls and spatula are completely dry. Melt butter in a wide skillet over medium-low heat. Add chopped kataifi and stir frequently for 8–10 minutes, until evenly deep golden and crisp, lowering heat if it browns too fast.

Spread toasted kataifi on a plate and let cool completely, about 10 minutes. Mix with pistachio cream, tahini and salt. The mixture should be thick and coated, not hot or watery.

Melt chopped chocolate in a dry microwave-safe bowl at 50% power in 20-second intervals, stirring each time, until just smooth. Stop heating while a few small pieces remain and stir until melted. Do not add water.

Brush or spoon about two-thirds of the chocolate into the bottoms and sides of two deep bar molds, leaving enough for sealing. Chill about 10 minutes until firm. Patch thin spots with a little reserved chocolate.

Divide the cool filling between the shells, pressing gently and leaving about 1/8 inch at the top for sealing. Rewarm the remaining chocolate very briefly if needed, spread over the filling and connect it to the shell edges. Tap molds gently to settle.

Refrigerate 25 minutes or until completely firm. Unmold carefully. Add optional chopped pistachios and a light pistachio-cream drizzle. Briefly rest at room temperature before cutting each bar into 6 pieces with a dry knife.

Store covered in the refrigerator for up to 5 days for best texture, or follow a shorter storage instruction on the pistachio cream. Keep cool until serving. This untempered chocolate will soften at warm room temperature.