01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, cream butter and powdered sugar until light and fluffy. Add egg yolk and vanilla extract; mix until fully incorporated.
03 - Sift in all-purpose flour and salt. Stir until a soft dough forms.
04 - Roll dough into 1-inch balls. Arrange on prepared baking sheets, spacing 2 inches apart. Flatten each ball gently with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, or until the edges are lightly golden. Cool completely on a wire rack.
06 - Pulse pistachios and granulated sugar in a food processor until finely ground. Blend in 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 onto the flat side of half the cookies. Sandwich with the remaining cookies.
08 - Roll the edges of the filled cookies in chopped pistachios if desired.
09 - Store in an airtight container at room temperature for up to 3 days.