01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat softened butter and powdered sugar with a mixer until light and fluffy. Add egg yolk and vanilla extract, then mix until well combined.
03 - Sift in all-purpose flour and salt. Mix until a soft dough develops.
04 - Roll dough into 1-inch balls and arrange on prepared baking sheets, spaced 2 inches apart. Gently flatten each ball with your palm or the back of a glass.
05 - Place baking sheets in oven and bake for 10 to 12 minutes, or until cookie edges are just turning golden. Transfer cookies to wire rack and allow to cool completely.
06 - In a food processor, blend pistachios and granulated sugar until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until smooth, adding an extra tablespoon of cream if necessary for a spreadable texture.
07 - Spread a dollop of pistachio cream onto the flat side of half the cooled cookies. Top with remaining cookies to form sandwiches.
08 - Optional: Roll the edges of filled cookies in finely chopped pistachios. Store cookies in an airtight container at room temperature for up to 3 days.