01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat butter with powdered sugar until light and fluffy. Incorporate egg yolk and vanilla; mix until smooth.
03 - Sift flour and salt into the creamed mixture. Stir until a soft dough develops.
04 - Roll dough into 1-inch balls and position evenly on prepared baking sheets, spacing 2 inches apart. Flatten gently with your palm or glass bottom.
05 - Bake 10 to 12 minutes, just until edges are barely golden. Transfer to a wire rack and cool completely.
06 - Pulse pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, butter, and 2 tablespoons heavy cream, processing until smooth. Add an additional tablespoon of cream, if needed, for spreadable texture.
07 - Spread pistachio cream onto the base of half the cookies. Place remaining cookies on top to form sandwiches.
08 - If desired, roll edges of filled cookies in chopped pistachios.
09 - Store in an airtight container at room temperature for up to 3 days.