01 - Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
02 - Cream together butter and powdered sugar in a large bowl until mixture is light and fluffy.
03 - Add egg yolk and vanilla extract; mix until fully combined.
04 - Sift flour and salt into the bowl and mix just until a soft dough forms.
05 - Roll dough into 1-inch balls, place on prepared baking sheets 2 inches apart, and press each gently to flatten.
06 - Bake cookies for 10 to 12 minutes, until edges begin to turn golden. Cool completely on a wire rack.
07 - Pulse pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until smooth, adding an extra tablespoon of cream if needed to create a spreadable consistency.
08 - Spread a generous dollop of pistachio cream onto the flat side of half the cookies. Top with remaining cookies to form sandwiches.
09 - Optional: Roll edges of filled cookies in finely chopped pistachios for decoration.
10 - Store cookies in an airtight container at room temperature for up to 3 days.