01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat together the unsalted butter and powdered sugar using a hand or stand mixer until light and fluffy. Blend in the egg yolk and vanilla extract until smooth.
03 - Sift in the all-purpose flour and salt. Mix until a soft, cohesive dough forms.
04 - Roll dough into 1-inch balls. Space dough balls 2 inches apart on prepared baking sheets. Gently flatten each ball with the palm or back of a glass.
05 - Bake for 10 to 12 minutes or until the edges are just turning golden. Cool cookies completely on a wire rack.
06 - Pulse shelled 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 and creamy, adding an extra tablespoon of cream if necessary for a spreadable texture.
07 - Spread pistachio cream generously onto the flat side of half the cookies. Top with the remaining cookies to create sandwiches.
08 - Roll the edges of filled cookies in finely chopped pistachios for a decorative finish.
09 - Place assembled cookies in an airtight container and store at room temperature for up to 3 days.