Preheat oven to 325°. Cream butter and confectioners' sugar until light and fluffy. Beat in vanilla. In another bowl, whisk flour and salt; gradually beat into creamed mixture. Add pistachios; mix well., Shape dough into 1-in. balls. Place 1 in. apart on ungreased baking sheets. Press a deep indentation in center of each with your thumb; fill each with 1/2 teaspoon jam., Bake until bottoms are light brown, 13-16 minutes. Remove from pans to wire racks to cool. If desired, dust with additional confectioners' sugar.