In a small bowl, combine flour and salt; set aside. In a medium mixing bowl, beat the 3/4 cup butter and 1/2 cup powdered sugar with an electric mixer on medium to high speed until combined. Beat in egg until combined. Beat in flour mixture on low speed just until combined. Cover and chill dough about 1 hour or until easy to handle.

Preheat oven to 350 degrees F. Shape dough into 3/4-inch balls. Place balls on an ungreased cookie sheet. Dip the bottom of a glass in granulated sugar and use to flatten each ball to a 1-1/2-inch circle. Bake about 8 minutes or until bottoms are lightly browned. Transfer cookies to a wire rack; cool.

Meanwhile, for filling, in a small mixing bowl, beat together 1/2 cup powdered sugar, the 1/4 cup butter, and rum on medium to high speed until combined.

To assemble cookies, spread filling on the flat sides of half of the cookies. Top with remaining cookies, flat sides down. Chill cookies about 30 minutes or until fillling sets. Drizzle or spread melted chocolate on tops of cookies and sprinkle with pistachio nuts. Chill about 10 minutes or until chocolate sets. Makes about 30 sandwich cookies.

Make-Ahead Tip: Prepare and bake cookies as directed; cool (do not fill or decorate). Place cookies in layers separated by waxed paper in an airtight container. Store at room temperature for up to 3 days or freeze for up to 3 months. Thaw frozen cookies, covered, at room temperature. Fill and decorate cookies.