01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat together butter and powdered sugar with a mixer until light and fluffy. Incorporate egg yolk and vanilla extract, mixing thoroughly.
03 - Sift in all-purpose flour and salt. Mix until a soft, cohesive dough forms.
04 - Roll dough into 1-inch balls. Arrange on prepared baking sheets, leaving 2 inches between each. Gently flatten each ball with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until the edges are just golden. Transfer to a wire rack and 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; process until smooth. Add an extra tablespoon of heavy cream if necessary for spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Top with the remaining cookies to form sandwiches.
08 - Roll edges of filled cookies in chopped pistachios for a decorative finish.
09 - Store cookies in an airtight container at room temperature for up to 3 days.