01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat unsalted butter and powdered sugar together with a mixer until light and airy. Add egg yolk and vanilla; blend thoroughly.
03 - Sift all-purpose flour and salt into the butter mixture. Mix until just combined and a soft dough forms.
04 - Roll dough into 1-inch balls. Place on prepared baking sheets about 2 inches apart. Gently flatten each ball with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until cookie edges are lightly golden. Remove from oven and allow to cool completely on a wire rack.
06 - Process pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream; process until smooth, adding up to 1 more tablespoon of cream for a spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Top each with a second cookie to form a sandwich.
08 - If desired, roll cookie edges in finely chopped pistachios for garnish.
09 - Store finished cookies in an airtight container at room temperature for up to 3 days.