01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat softened butter and powdered sugar until light and aerated. Incorporate egg yolk and vanilla extract, blending thoroughly.
03 - Sift in all-purpose flour and salt. Mix until a cohesive, soft dough forms.
04 - Portion dough into 1-inch balls. Set onto prepared baking sheets, leaving 2 inches between each. Press gently to flatten using 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 - Process pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, butter, and 2 tablespoons of heavy cream; blend to a smooth, spreadable cream, using a third tablespoon of cream if needed for consistency.
07 - Spread pistachio cream on the flat side of half the cooled cookies. Gently sandwich with remaining cookies.
08 - If desired, roll the cream edges of each cookie in chopped pistachios for a decorative finish.
09 - Transfer to an airtight container and store at room temperature for up to 3 days.