01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - Combine softened butter and powdered sugar in a large mixing bowl; blend with a mixer until light and fluffy.
03 - Add the egg yolk and vanilla extract to the butter mixture; beat thoroughly until smooth.
04 - Sift in all-purpose flour and salt. Mix until a soft dough forms without overworking.
05 - Roll dough into 1-inch balls. Arrange on prepared baking sheets, spacing 2 inches apart. Gently flatten each with your palm or the base of a glass.
06 - Bake for 10 to 12 minutes, or until the edges show a hint of golden color. Transfer to a wire rack and cool completely.
07 - Pulse pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, softened butter, and two tablespoons of heavy cream; blend until smooth, adding an additional tablespoon of cream if a spreadable texture is needed.
08 - Spread pistachio cream onto the flat side of half the cookies. Top with the remaining cookies to create sandwiches.
09 - Roll the edges of each sandwich cookie in finely chopped pistachios for an elegant finish.
10 - Store cookies in an airtight container at room temperature for up to three days.