In a large bowl, beat butter with a mixer at medium speed until smooth, about 1 minute. Add confectioners’ sugar, vanilla, and salt; beat just until combined. With mixer on low speed, gradually add flour until combined. Stir in ¾ cup pistachios.
On a lightly floured surface, roll dough into a 10x2-inch log. Wrap dough tightly in parchment paper. Refrigerate for at least 1 hour or overnight.
Preheat oven to 350°. Line 2 large rimmed baking sheets with parchment paper.
Brush outside of dough log with egg yolk, and roll in remaining ¼ cup pistachios until fully coated. Slice into ¼-inch rounds, and place 2 inches apart on prepared pans.
Bake until golden brown, about 12 minutes. Let cool completely on wire racks.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/pistachio-sandies-recipe/