01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Combine softened butter and powdered sugar in a large mixing bowl; beat until light and fluffy. Incorporate the egg yolk and vanilla extract, mixing thoroughly.
03 - Sift all-purpose flour and salt into wet mixture. Mix gently until a soft, cohesive dough is formed.
04 - Roll dough into 1-inch balls. Arrange on prepared baking sheets with 2-inch spacing. Flatten each gently with your palm or the base of a glass.
05 - Bake in preheated oven for 10 to 12 minutes until edges are just turning golden. Cool completely on a wire rack.
06 - Pulse pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until smooth, adding additional cream for spreadable consistency if needed.
07 - Spread pistachio cream onto the flat side of half of the cooled cookies. Top with remaining cookies to form sandwiches.
08 - If desired, roll cookie edges in finely chopped pistachios.
09 - Store cookies in an airtight container at room temperature for up to three days.