Place the popcorn in a bowl large enough to toss it easily.
Line two baking sheets with Silpats or parchment paper.
Slowly add the melted chocolate, which should not be too hot, one quarter at a time, folding the chocolate into the popcorn with a spatula, and carefully scraping the sides of the bowl to coat all pieces evenly.
Spread the popcorn on the prepared baking sheets.
Using a small strainer, sprinkle the cocoa powder little by little over the surface of the popcorn. Stir carefully as you sift, breaking the popcorn into clusters. Continue adding cocoa powder until all kernels are well coated.
Cover with plastic wrap and refrigerate for about 20 minutes or until the chocolate is set.
Remove from refrigerator and store in an airtight container at room temperature for up to 2 days.
To serve, place in a serving bowl and sprinkle with pistachios, if desired.