Melt 2 tablespoons of butter in a nonstick skillet over medium heat, then add the chopped kataifi. Stir it often so every strand toasts evenly and turns a deep golden color. This should take about 8 to 10 minutes, and your kitchen will smell incredible.
Transfer the toasted pastry to a bowl right away while it's still warm. Stir in the pistachio cream, tahini, and salt until everything is coated and combined. The filling should look glossy and hold together loosely when you press it with a spoon.
Set up a double boiler and melt the white chocolate chips until smooth. Stir in a drop or two of green food coloring if you're using it, then drizzle it across the bottom of your mold in loose, irregular lines. Pop the mold in the freezer for a few minutes to set.
Melt most of your milk chocolate with the remaining butter, then spread a thin layer over the bottom and sides of the mold. Freeze this layer until it's firm to the touch, which usually takes about 10 minutes. This shell is what gives the bar its clean snap.
Spoon the pistachio filling into the mold and press it into an even layer with your spatula. Pour the rest of the melted chocolate over the top, then scrape across the surface with your bench scraper to level everything out and remove excess.
Refrigerate the bars for at least 40 minutes, or until they're completely firm. Once set, pop them out of the mold gently and trim any uneven edges with a sharp knife before serving.