01 - Preheat the 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 with a mixer until light and fluffy. Add the egg yolk and vanilla extract, mixing until fully incorporated.
03 - Sift in the all-purpose flour and salt. Mix on low until a soft dough forms.
04 - Roll the dough into 1-inch balls and arrange on the prepared baking sheets, spacing each about 2 inches apart. Flatten gently with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until edges begin to turn 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 heavy cream. Process until smooth, adding an extra tablespoon cream if needed for a spreadable texture.
07 - Spread pistachio cream onto the flat side of half the cookies. Top with remaining cookies to form sandwiches.
08 - If desired, roll the edges of the filled cookies in finely chopped pistachios. Store cookies in an airtight container at room temperature for up to 3 days.