01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
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.
03 - Add the egg yolk and vanilla extract to the butter mixture and blend until fully combined.
04 - Sift in the all-purpose flour and salt. Mix until a soft, cohesive dough forms without overworking.
05 - Roll the dough into 1-inch balls and place them on the lined baking sheets, spacing each ball 2 inches apart. Gently flatten each with your palm or the back of a glass.
06 - Bake the cookies for 10 to 12 minutes until edges show slight golden color. Cool completely on a wire rack.
07 - In a food processor, pulse pistachios and granulated sugar until very finely ground. Add the melted white chocolate, softened butter, and 2 tablespoons heavy cream, then process until smooth. Blend in an extra tablespoon of cream if needed for desired consistency.
08 - Spread a generous layer of pistachio cream on the flat side of half the cookies. Top each with a second cookie to form a sandwich.
09 - Roll the edges of the filled cookies in finely chopped pistachios if desired.
10 - Store cookies in an airtight container at room temperature for up to 3 days.