Preheat your oven to 375°F (190°C). Line a baking sheet with a parchment paper.
Roll out the thawed puff pastry sheet on a lightly floured surface. Cut the pastry into strips about 1 inch wide.
Wrap each strip of puff pastry around the molds or a cone-shaped object, starting from the tip and wrapping towards the wider end. Brush it with egg wash.
Bake the pastry horns in the preheated oven for 18-20 minutes, or until they are golden brown and crispy. Remove from the oven and let them cool completely.
Once the pastry horns are cool, carefully remove them from the molds.
In a mixing bowl, whip heavy cream with powdered sugar and pistachio butter until stiff peaks form.
Fill each horn with the whipped cream filling using a piping bag or spoon.
Dip the edges in crushed pistachios.