Preheat your oven to 375°F (190°C). Grease a muffin tin or line it with muffin liners.
In a small bowl, mix together the chopped pistachios, granulated sugar, and ground cinnamon. Set aside.
Roll out the puff pastry sheet on a lightly floured surface to smooth out creases and slightly enlarge it.
Brush the melted butter evenly over the entire surface of the puff pastry sheet.
Sprinkle the prepared pistachio mixture evenly over the buttered puff pastry sheet, pressing gently to adhere.
Starting from one of the longer edges, tightly roll up the puff pastry sheet like a jelly roll.
Using a sharp knife, cut the roll crosswise into 12 equal slices.
Place each slice, cut side up, into the prepared muffin tin. Gently press down on each slice to slightly flatten.
Bake in the preheated oven for 20-25 minutes, or until the muffins are golden brown and puffed up.
Remove from the oven and allow the muffins to cool in the tin for a few minutes before transferring to a wire rack to cool completely.
Optional: Dust with powdered sugar before serving.
Serve the pistachio croissant muffins warm or at room temperature. They are best enjoyed fresh but can be stored in an airtight container for up to 2 days.