Line a baking sheet with waxed paper. Put pistachios in a shallow bowl.
Melt chocolate (see "Melting Chocolate: Be Gentle," below). Holding at one end, dip 1 marshmallow at a time into the chocolate, turning to coat about 3/4 of the marshmallow. Use a knife to scrape excess chocolate off the bottom and back into the bowl. Hold marshmallow over the bowl of pistachios and sprinkle the nuts over the chocolate.
Put marshmallow (nut side down) on prepared baking sheet and repeat dipping, sprinkling, and placement with each remaining marshmallow. Chill marshmallows until chocolate is set, about 1 hour. Store between sheets of waxed paper in an airtight container in the refrigerator for up to 2 weeks.
Melting chocolate: Be gentle. Fill a saucepan with 2 to 3 in. of water. Put finely chopped chocolate in a heatproof bowl that will fit over pan (the bottom of the bowl should not touch the water); set bowl aside. Bring water to a boil, turn off heat, then set bowl over pan. Use a heatproof spatula or wooden spoon to stir the chocolate frequently and encourage even melting. Once the chocolate is melted, stir well and use immediately.
Note: Nutritional analysis is per marshmallow.