01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, cream together unsalted butter and powdered sugar with a hand or stand mixer until light and fluffy. Blend in the egg yolk and vanilla extract thoroughly.
03 - Sift in all-purpose flour and salt. Mix to combine until a soft dough forms.
04 - Roll dough into 1-inch balls and arrange them on the lined baking sheets, spacing 2 inches apart. Flatten each ball gently with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes or until the edges are just turning golden. Remove and allow cookies to cool 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. Process until smooth, adding an additional tablespoon of cream if required for spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Place remaining cookies on top to form sandwiches.
08 - Roll the edges of filled cookies in finely chopped pistachios for garnish, if desired.
09 - Store the cookies in an airtight container at room temperature for up to 3 days.