1. In a food processor, process butter, confectioners' sugar, and salt until smooth. Add flour and pulse until a dough forms. Stir in pistachios. Shape dough into two 8-inch-long logs, tightly wrap in plastic, and freeze until firm, 30 minutes (or up to 1 month).

2. Preheat oven to 350 degrees, with racks in upper and lower thirds. Cut dough into 1/4-inch slices and transfer to parchment-lined baking sheets. Bake until cookies are set and barely golden at edges, 10 to 15 minutes, rotating sheets halfway through. Let cool completely on sheets on wire racks.

3. Spread 1/2 teaspoon jam on bottoms of half the cookies, then sandwich with remaining cookies.