01 - Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
02 - In a large mixing bowl, blend unsalted butter and powdered sugar until light and fluffy using a hand or stand mixer. Incorporate egg yolk and vanilla extract until thoroughly combined.
03 - Sift all-purpose flour and salt into the butter mixture and mix until a soft dough forms.
04 - Portion dough into 1-inch balls, spacing them 2 inches apart on prepared baking sheets. Gently flatten each with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, until the edges are lightly golden. Cool completely on a wire rack.
06 - Pulse pistachios and granulated sugar in a food processor until finely ground. Blend in melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until creamy, adding up to 1 extra tablespoon cream for spreadable consistency.
07 - Spread a layer of pistachio cream on the flat side of half the cookies and sandwich with the remaining cookies.
08 - Roll the edges of filled cookies in finely chopped pistachios if desired.
09 - Store in an airtight container at room temperature for up to 3 days.