01 - Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
02 - In a large bowl, beat unsalted butter and powdered sugar until light and fluffy. Incorporate egg yolk and vanilla extract, mixing until well combined.
03 - Sift all-purpose flour and salt over the wet mixture. Mix until a soft dough forms.
04 - Roll dough into 1-inch balls and arrange on prepared baking sheets, maintaining 2-inch spacing. Flatten each ball gently with your palm or a glass.
05 - Bake for 10 to 12 minutes, or until the edges are just turning golden. Allow cookies to cool completely on a wire rack.
06 - In a food processor, pulse shelled pistachios with granulated sugar until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until smooth, adding an extra tablespoon cream for spreadable texture if necessary.
07 - Spread pistachio cream onto the flat side of half the cookies. Top with remaining cookies to assemble into sandwiches.
08 - If desired, roll the edges of the filled cookies in finely chopped pistachios. Store cookies in an airtight container at room temperature for up to 3 days.