01 - Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
02 - In a large mixing bowl, cream together the softened unsalted butter and powdered sugar using a hand or stand mixer until light and fluffy. Add the egg yolk and vanilla extract, mixing thoroughly.
03 - Sift in all-purpose flour and salt. Mix until a soft, cohesive dough forms without overworking.
04 - Roll dough into 1-inch balls and space them 2 inches apart on prepared sheets. Gently flatten each ball with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until edges are just lightly golden. Allow cookies to 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 mixture is smooth and spreadable, adding an extra tablespoon of cream if needed.
07 - Spread a generous amount of pistachio cream onto the flat side of half the cookies. Top with the remaining cookies to form sandwiches.
08 - Optional: Roll the edges of the filled cookies in finely chopped pistachios for a decorative finish.
09 - Transfer finished cookies to an airtight container and store at room temperature for up to 3 days.