Melt the Chocolate: In a microwave-safe bowl, combine dark chocolate chips and 1 tablespoon coconut oil. Microwave in 20-second intervals, stirring in between, until completely melted and smooth.
Prepare the Chocolate Molds: Spoon 2 tablespoons of melted chocolate into each chocolate mold. Use a spoon or silicone brush to spread the chocolate evenly over the bottom and sides of the molds. Place the molds in the refrigerator for about 10 minutes to set.
Cook the Kataifi Pastry: While the chocolate is setting, chop the Kataifi (shredded phyllo dough) into small pieces. Heat 1 tablespoon of coconut oil (or butter) in a pan over medium heat. Add the Kataifi and cook, stirring frequently, until golden brown and crispy.
Make the Pistachio Filling: Remove the pan from the heat and stir in pistachio butter and tahini paste (if using). Mix until fully combined and smooth.
Fill the Molds: Once the chocolate in the molds has set, spoon the pistachio filling into each mold, spreading it evenly.
Top with Chocolate: Add the remaining melted chocolate on top of the pistachio filling in each mold, spreading it evenly with a spoon.
Set the Bars: Return the molds to the refrigerator or freezer for 20 minutes (or 10 minutes in the freezer) to allow the bars to fully set.
Serve: Once set, carefully remove the bars from the molds and enjoy!