Melt the butter in a large non-stick pan over low heat. When melted, add the roughly chopped kadayif and stir. Toast the kadayif until deeply golden and crunchy, stirring occasionally, about 8-10 minutes depending on the heat of your stove. Keep an eye on it and don’t forget to stir at regular intervals so the kadayif toasts evenly. When toasted, remove from the pan and let cool.
While the kadayif is cooling, melt the chopped chocolate in a double boiler. Add an inch of water to a small pot and place it on the stove. Place a larger, heat-proof bowl into the pot - the rim of the pot should support the bowl so that the bottom of the bowl doesn’t touch the water. Place two-thirds of the chopped chocolate in the bowl and turn the heat on to low. Stir the chocolate with a silicone spatula to encourage even melting.
When the chocolate is smooth and melted, carefully remove it from the double boiler and add the remaining chocolate. Stir until all the chocolate is smooth.
If desired, optionally melt white chocolate and color with oil-based green food coloring. Gently splatter into the chocolate bar mold before adding the chocolate for the shell.
Add about 1/4 cup of melted chocolate (per bar) to the chocolate bar mold and tip the mold to spread the chocolate evenly across the bottom, sides, and corners. Pour out any excess chocolate back into your bowl of melted chocolate and scrape off any excess with an offset spatula. Let the chocolate set completely by chilling in the fridge or freezer until hard.
While the chocolate is setting, make the filling. Add the pistachio cream, tahini, and a pinch of salt to a large bowl. Mix until smooth then fold in the toasted kadayif until completely combined.
Take the chocolate bar mold and fill evenly with the pistachio filling to about 3/4 way up the mold.
Gently remelt the chocolate then evenly pour over the filling, using an offset spatula to spread evenly. Place the chocolate bars in the fridge or freezer to set completely.
Unmold and enjoy!