Toast the Kataifi- Melt butter in a large skillet with high sides over medium heat. Add kataifi and cook, stirring often, until golden brown and crispy (about 10 minutes). Set aside to cool.
Prepare the Pistachio Filling- In a bowl, mix pistachio cream, tahini, and salt until smooth. Stir in the toasted and cooled katafi.
Temper the Chocolate (Optional but highly recommended) Microwave Method: Chop the chocolate finely. Place 2/3 of it in a microwave-safe bowl. Microwave in 20–30 second intervals, stirring well between each, until melted and reaching 115°F-120°F (46-48°C) for dark chocolate or 110-115°F (43-46°C) for milk chocolate.
Stir in the remaining 1/3 chocolate to cool to 89°F (31°C) for dark or 86°F (30°C) for milk chocolate.
Double Broiler Method: Melt 2/3 of the chocolate in a double boiler until it reaches the correct temperature. For dark chocolate, 115°F-120°F (46-48°C), and for milk chocolate, 110-115°F (43-46°C)
Remove from heat. Stir in the remaining 1/3 of the chocolate to cool to 89°F (31°C) for dark chocolate or 86°F (30°C) for milk chocolate.
Assemble the Bars- Using a pastry brush or the back of a spoon, coat the inside of candy bar molds with a thin layer of tempered chocolate. Chill for about 15 minutes, or until set.
Spoon in the pistachio/katafi mixture, half in each, and spread evenly.
Pour the remaining chocolate over the top and smooth the surface. Make sure the chocolate is at the correct temperature (89°F for dark or 86°F for milk/white). If needed, gently rewarm it in 5-second bursts until it reaches 89°F (32°C) for dark chocolate or 86°F (30°C) for milk/white chocolate.
Chill Bars- Refrigerate for 20 minutes or until chocolate is firm.
Unmold and Serve- Remove bars from molds and slice if desired.