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. Blend in egg yolk and vanilla until fully incorporated.
03 - Sift in flour and salt. Mix until a soft dough forms without overworking.
04 - Portion dough into 1-inch balls and arrange on prepared baking sheets, leaving 2 inches between each. Gently flatten each ball with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until the edges turn just golden. Allow to cool completely on a wire rack.
06 - Pulse shelled pistachios and granulated sugar in a food processor until finely ground. Add melted white chocolate, softened butter, and 2 tablespoons heavy cream; blend until smooth. Add extra cream if needed for spreadable consistency.
07 - Spread a layer of pistachio cream onto the flat side of half the cookies. Sandwich with the remaining cookies.
08 - If desired, roll edges of the assembled cookies in finely chopped pistachios for decoration.
09 - Transfer cookies to an airtight container and store at room temperature for up to three days.