01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat softened butter and powdered sugar with a mixer until light and fluffy. Incorporate egg yolk and vanilla extract, mixing thoroughly.
03 - Sift all-purpose flour and salt into the mixture. Blend on low speed until a soft dough forms.
04 - Roll dough into 1-inch diameter balls and arrange on prepared baking sheets, spacing each about 2 inches apart. Gently flatten each ball with your palm or the back of a glass.
05 - Bake cookies for 10 to 12 minutes until the edges are just golden. Transfer to a wire rack and let cool completely.
06 - In a food processor, pulse pistachios and granulated sugar until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons of heavy cream. Process until smooth, adding an extra tablespoon of cream if needed to achieve a spreadable texture.
07 - Spread pistachio cream onto the flat sides of half the cookies. Sandwich with the remaining cookies, pressing gently.
08 - Roll the edges of filled cookies in chopped pistachios for an elegant finish.
09 - Store filled cookies in an airtight container at room temperature for up to 3 days.