01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat the butter and powdered sugar together until light and fluffy. Add egg yolk and vanilla extract; mix until fully incorporated.
03 - Sift in the all-purpose flour and salt. Mix until a soft, cohesive dough forms.
04 - Roll dough into 1-inch balls and arrange on prepared sheets, spacing them 2 inches apart. Gently flatten each ball with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, until edges are just turning golden. Cool cookies completely on a wire rack.
06 - In a food processor, pulse pistachios and granulated sugar until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons of heavy cream; blend until smooth. Add an extra tablespoon of cream if needed for a spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cooled cookies. Press remaining cookies on top to form sandwiches.
08 - Roll the edges of each sandwich in finely chopped pistachios if desired.
09 - Store in an airtight container at room temperature for up to 3 days.