01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat butter and powdered sugar until light and fluffy. Add egg yolk and vanilla extract, mixing until combined.
03 - Sift all-purpose flour and salt into the bowl. Mix until a soft dough forms.
04 - Roll dough into 1-inch balls and arrange on prepared sheets, spaced 2 inches apart. Gently flatten each ball with your palm or the bottom of a glass.
05 - Bake for 10 to 12 minutes, or until the edges turn golden. Transfer to a wire rack and allow to cool completely.
06 - In a food processor, pulse pistachios and granulated sugar until finely ground. Add melted white chocolate, butter, and 2 tablespoons heavy cream; blend until smooth, adding extra cream if needed for spreadable texture.
07 - Spread pistachio cream onto the underside of half the cookies. Top with remaining cookies to form sandwiches.
08 - Roll edges of filled cookies in finely chopped pistachios, if desired.
09 - Store cookies in an airtight container at room temperature for up to 3 days.