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 with a mixer until light and fluffy. Add the egg yolk and vanilla extract; mix until combined.
03 - Sift in the all-purpose flour and salt. Stir until a soft, cohesive dough forms.
04 - Portion dough into 1-inch balls and arrange on prepared baking sheets, spacing 2 inches apart. Gently flatten each ball with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until edges begin to turn 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 of heavy cream. Blend until smooth, adding an extra tablespoon of cream if needed to achieve a spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Sandwich with the remaining cookies, pressing gently.
08 - Roll the edges of the filled cookies in finely chopped pistachios for garnish, if desired.
09 - Place cookies in an airtight container and store at room temperature for up to 3 days.