01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, cream together softened butter and powdered sugar with a 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 smooth, soft dough forms.
04 - Roll dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets. Flatten each lightly with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until edges are lightly 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 an additional tablespoon of cream if needed for spreadable consistency.
07 - Spread a layer of pistachio cream on the flat side of half the cookies. Top with the remaining cookies to create sandwiches.
08 - Optional: Roll the edges of filled cookies in finely chopped pistachios for decoration.
09 - Store cookies in an airtight container at room temperature for up to 3 days.