01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, blend the unsalted butter and powdered sugar using a mixer until mixture is light and fluffy. Add the egg yolk and vanilla extract; mix thoroughly.
03 - Sift in the all-purpose flour and salt. Mix gently until a soft, cohesive dough forms.
04 - Roll dough into 1-inch balls and arrange them on prepared baking sheets, spacing 2 inches apart. Flatten each ball gently with your hand or the base of a glass.
05 - Bake for 10 to 12 minutes or until the edges appear golden. Remove from the oven and allow cookies to cool completely on a wire rack.
06 - In a food processor, pulse the pistachios and granulated sugar until finely ground. Add melted white chocolate, softened butter, and two tablespoons heavy cream; process until smooth, adding up to one additional tablespoon cream if necessary for a spreadable consistency.
07 - Spread pistachio cream onto the flat side of half the cookies. Sandwich with remaining cookies.
08 - Roll the edges of filled cookies in finely chopped pistachios for an elegant presentation.
09 - Place completed cookies in an airtight container and store at room temperature for up to three days.