Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
In a mixing bowl, cream together the butter and granulated sugar until light and fluffy.
Add the egg yolk and vanilla extract; mix until well combined.
In another bowl, sift together the flour, cocoa powder, and salt before gradually incorporating into the wet mixture.
Form dough balls and press your thumb into each to create indentations.
Roll in chopped pistachios if desired.
Bake for 10-12 minutes or until edges are set. Let cool slightly before pressing the centers again.
Prepare the filling by melting white chocolate, then mixing it with pistachio butter and powdered sugar until smooth. Fill each cookie indentation with this mixture.