01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat unsalted butter and powdered sugar until light and fluffy. Add the egg yolk and vanilla extract, blending thoroughly.
03 - Sift all-purpose flour and salt over the creamed mixture. Mix just until a soft dough forms.
04 - Roll dough into 1-inch balls and arrange 2 inches apart on prepared baking sheets. Gently flatten each ball with your palm or the base of a glass.
05 - Bake for 10 to 12 minutes until the edges are lightly golden. Transfer to a wire rack and allow to cool completely.
06 - In a food processor, blend pistachios and granulated sugar until finely ground. Add melted white chocolate, unsalted butter, and 2 tablespoons heavy cream, processing until smooth. If necessary, add more cream to achieve a spreadable consistency.
07 - Spread pistachio cream on the flat side of half the cooled cookies. Top with the remaining cookies to form sandwiches.
08 - Roll the edges of the sandwiched cookies in finely chopped pistachios for garnish.
09 - Store assembled cookies in an airtight container at room temperature for up to 3 days.