Make the pistachio cream: In a food processor, combine the pistachios, sugar, olive oil, vanilla and salt. Process until it becomes a creamy nut butter, stopping and scraping down the sides of the food processor with a spatula as you go to ensure a uniform mixture. The pistachio mixture will progress from chopped nuts to a thick nut paste, then finally transform into a silky, slightly thin nut butter consistency; this takes about 6 minutes.

Add the tahini and process until fully incorporated, about 30 seconds more. Keep the mixture at room temperature while you make the rest of the filling.

Toast the kataifi: In a large high-sided skillet or large pot over medium heat, melt the butter. When it starts to bubble, add the chopped kataifi and cook, stirring frequently and breaking up any unbrowned clumps, until it is evenly crisp and golden brown, about 15 minutes. Transfer to a large plate and let cool completely, about 10 minutes. Once cooled, combine the toasted kataifi with the pistachio cream in a large bowl, mixing with a spatula until fully combined. Keep it at room temperature while you work with the chocolate.

Melt the chocolate: Set a medium heatproof bowl over a pot of simmering water. Add half the chocolate to the bowl and heat, stirring occasionally, until melted. (Alternatively, melt chocolate in a microwave-safe bowl, microwaving in 15-second increments and stirring well between intervals, until smooth.)

Once the chocolate in the bowl is fully melted, transfer the bowl to a work surface. (Keep the pot of hot water covered in case it’s needed later.) Working with a small handful at a time, begin adding the unmelted chocolate, stirring constantly until each addition is fully melted before adding more. (This process of melting chocolate and then stirring in unmelted chocolate is tempering, and is essential for creating chocolate bars with shine and snap.)

Assemble the chocolate bars: Place four 6-by-3-by-1-inch silicone chocolate bar molds on a sheet pan so it’s easy to transfer them in and out of the fridge. Working with one chocolate bar mold at a time, place ¼ cup melted chocolate in a mold; using a pastry brush or the back of a small spoon, paint the inside of each mold with the chocolate, completely covering the bottom and sides. Transfer the molds to the fridge until the chocolate is hard, at least 30 minutes.

Once the chocolate is fully set, remove the molds from the fridge. Divide the filling evenly among the molds (about ¾ cup loosely packed filling per mold). Smooth the filling out with a spatula to fill the molds almost to the tops in an even layer, and transfer to the fridge until the kataifi mixture has firmed up slightly, about 10 minutes.

Give the remaining melted chocolate a stir (if it’s become too thick to stir, set the bowl over the pot of water over low heat, stirring until easily spreadable, or microwave in 5-second intervals, stirring in between intervals). Pour the remaining chocolate on top of the bars, dividing evenly. Using an offset spatula or the flat side of a knife, spread the chocolate out to cover the entire surface of the filling in a smooth, even layer. Scrape any excess chocolate off the tops of the molds with the spatula or knife. This will ensure clean edges on the bars when you unmold them.

Return the chocolate bars to the fridge and chill until hard, 30 to 45 minutes. To unmold, gently pull the silicone away from each bar and pop them out one by one. Enjoy right away, or wrap the bars individually to keep in the fridge for up to 3 weeks. Enjoy them chilled or let them come to room temperature before snapping into one.