01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - In a large mixing bowl, cream together the softened butter and powdered sugar using a mixer until light and fluffy. Add egg yolk and vanilla extract; incorporate thoroughly.
03 - Sift in all-purpose flour and salt. Mix until a soft, cohesive dough forms.
04 - Roll dough into 1-inch balls and set on prepared baking sheets 2 inches apart. Flatten each ball slightly with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes until edges turn lightly golden. 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 heavy cream; process until smooth, adding additional cream if needed for a spreadable consistency.
07 - Spread pistachio cream on the flat side of half the cookies and sandwich with the remaining cookies.
08 - Roll edges of filled cookies in finely chopped pistachios if desired. Store in an airtight container at room temperature for up to 3 days.