01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, cream unsalted butter and powdered sugar until light and fluffy using a hand or stand mixer. Blend in the egg yolk and vanilla extract until well combined.
03 - Sift in all-purpose flour and salt. Mix until a soft dough forms without overworking the mixture.
04 - Roll dough into 1-inch balls and arrange on the prepared baking sheets, leaving 2 inches between each. Gently flatten each ball with your palm or the back of a glass.
05 - Bake the cookies for 10 to 12 minutes or until edges are just turning golden. Remove from oven and cool completely on a wire rack.
06 - In a food processor, pulse shelled pistachios and granulated sugar until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until smooth, adding an additional tablespoon of cream if needed to achieve a spreadable consistency.
07 - Spread a generous amount of pistachio cream onto the flat side of half the cookies. Top with remaining cookies to make sandwiches.
08 - Roll the edges of filled cookies in finely chopped pistachios for decoration if desired.
09 - Store completed cookies in an airtight container at room temperature for up to 3 days.