How to Make It
Preheat oven to 325°F. Beat butter with a stand mixer fitted with a paddle attachment on medium speed until creamy, about 2 minutes. Gradually add sugar, beating well. Stir together flour, salt, and 1/4 cup of the pistachios in a bowl. Gradually add to butter mixture, beating until just combined, 30 seconds. Divide dough into 3 equal portions.
Place 1 dough portion on a baking sheet lined with parchment paper. Roll or pat into a 6-inch circle. Score dough evenly into 8 triangles. Repeat procedure with remaining 2 portions on the same baking sheet.
Bake shortbread in preheated oven until barely golden, 23 to 25 minutes. Cool on baking sheet 5 minutes. Using a very sharp knife, cut shortbread into triangles following the scored lines in dough. Transfer shortbread to wire racks; cool completely, about 15 minutes.
Melt chocolate in a small microwavable bowl on MEDIUM (50%) power until melted and smooth, 2 1/2 to 3 minutes, stopping to stir every 30 seconds.
Carefully dip the wide edge of 1 shortbread piece into melted chocolate. Transfer to a sheet of wax paper; immediately sprinkle shortbread with some of the remaining 1/2 cup chopped pistachios and a small sprinkle of sea salt. Repeat procedure with remaining shortbread pieces, chocolate, pistachios, and salt. Let stand until chocolate is firm, about 30 minutes. Store in an airtight container up to 1 week.