01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat butter and powdered sugar until light and fluffy. Add egg yolk and vanilla extract; mix thoroughly.
03 - Sift in all-purpose flour and salt. Blend until a soft dough develops.
04 - Roll dough into 1-inch balls. Arrange on prepared baking sheets, 2 inches apart. Flatten each gently with your palm or the base of a glass.
05 - Bake for 10 to 12 minutes until edges are lightly golden. Transfer to a wire rack and cool completely.
06 - Pulse pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream; blend until smooth. Add additional cream if needed for spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Cover with remaining cookies to create sandwiches.
08 - Optional: Roll edges of filled cookies in finely chopped pistachios for a decorative finish.
09 - Store cookies in an airtight container at room temperature for up to three days.