01 - Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.
02 - In a large bowl, beat unsalted butter and powdered sugar using a hand or stand mixer until light and fluffy. Add the egg yolk and vanilla extract, mixing thoroughly.
03 - Sift in the all-purpose flour and salt. Mix just until a soft dough forms without overworking.
04 - Portion dough into 1-inch balls and arrange on prepared baking sheets, leaving a 2-inch gap between each. Flatten each ball gently with your palm or the bottom of a glass.
05 - Bake in the center of the oven for 10 to 12 minutes, or until edges begin to color lightly. Transfer to a wire rack and allow to cool completely.
06 - In a food processor, pulse shelled pistachios and granulated sugar until finely ground. Blend in melted white chocolate, softened butter, and 2 tablespoons of heavy cream. Process until mixture is smooth and spreadable, adding the extra tablespoon of cream if a thinner consistency is needed.
07 - Spread a dollop of pistachio cream on the flat side of half the cooled cookies. Sandwich with the remaining cookies, pressing gently.
08 - Roll edges of filled cookies in finely chopped pistachios for garnish, if desired.
09 - Store the filled cookies in an airtight container at room temperature for up to three days.