01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, cream together softened butter and powdered sugar until light and fluffy. Incorporate egg yolk and vanilla extract, mixing thoroughly.
03 - Sift together flour and salt. Gradually add to the butter mixture, blending just until a soft dough forms.
04 - Roll dough into 1-inch balls and arrange 2 inches apart on prepared baking sheets. Gently flatten each ball using your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until cookie edges begin to turn golden. Allow to 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 smooth, adding additional cream if necessary for a spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Top with remaining cookies to create sandwiches.
08 - Roll filled cookie edges in finely chopped pistachios for a decorative finish.
09 - Store in an airtight container at room temperature for up to three days.