01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat together unsalted butter and powdered sugar using a hand or stand mixer until light and fluffy. Incorporate the egg yolk and vanilla extract, mixing until fully combined.
03 - Sift in the all-purpose flour and salt. Mix just until a soft dough forms.
04 - Roll the dough into 1-inch balls and position them on the prepared baking sheets, spacing 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 the edges begin to turn golden. Cool completely on a wire rack.
06 - In a food processor, pulse 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 cream if needed for a spreadable consistency.
07 - Spread a generous layer of pistachio cream onto the flat side of half the cookies. Top with remaining cookies to form sandwiches.
08 - Optionally, roll the edges of the filled cookies in finely chopped pistachios for decoration.
09 - Place cookies in an airtight container and store at room temperature for up to 3 days.