Preheat the oven to 180°C (350°F).
Prepare a baking dish by greasing it with butter.
In a large bowl, combine the chopped kataifi, melted butter, and sugar.
Press the mixture evenly into the prepared baking dish.
Bake the kataifi base for 15-20 minutes until golden brown.
While the base is baking, melt the dark chocolate in a heatproof bowl over simmering water.
Add the chopped pistachios and vanilla extract to the melted chocolate, stirring to combine.
Pour the chocolate mixture evenly over the baked kataifi base.
Spread the chocolate gently with a spatula to cover the entire surface.
Let the bar cool at room temperature for about 30 minutes, then refrigerate until set.
Once set, slice into bars and serve.
