01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat together the softened butter and powdered sugar until the mixture becomes light and fluffy. Add the egg yolk and vanilla extract, mixing thoroughly.
03 - Sift the all-purpose flour and salt into the bowl. Mix until a soft dough forms, ensuring ingredients are evenly incorporated.
04 - Roll dough into 1-inch balls. Arrange on the prepared baking sheets, leaving 2 inches between each. Press each ball gently with your palm or the back of a glass to flatten.
05 - Bake for 10 to 12 minutes, or until the cookie edges show a light golden color. Transfer to a wire rack and allow cookies to cool completely.
06 - In a food processor, pulse the pistachios and granulated sugar until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream. Process until smooth, adding an additional tablespoon of cream if needed for a spreadable consistency.
07 - Spread pistachio cream on the flat side of half the cookies. Top with the remaining cookies, gently pressing to form sandwiches.
08 - If desired, roll the edges of each filled cookie in chopped pistachios for a decorative finish.
09 - Place cookies in an airtight container and store at room temperature for up to 3 days.