Line a baking sheet or plate with parchment or waxed paper; place in the freezer at least 15 minutes. Using an ice-cream scoop to shape, place 8 round scoops of ice cream (about 1/4 cup each) on baking sheet; freeze until firm, at least 30 minutes.
Place chocolate in a medium heatproof bowl set over (not in) a saucepan of simmering water. Stir frequently, until almost melted. Remove from heat, and stir until completely melted.
Spoon 1 heaping tablespoon of warm melted chocolate on 1 scoop of ice cream. Using bottom of spoon, spread chocolate over and around the sides; sprinkle with pistachios. Working quickly, repeat with remaining scoops. Freeze until firm, at least 5 minutes.
Cover in plastic wrap, and freeze up to 2 weeks.