Link a large rimmed baking sheet with a long sheet of parchment paper.
If desired, pulse the pistachios in a food processor until they are roughly chopped. You can also carefully chop them using a sharp knife on a cutting board, or leave them whole.
Transfer the chocolate chips to a microwave-safe bowl.
Microwave for 30-second intervals, stirring well in between each interval, until the chocolate chips are 90% of the way melted. This takes 4 (30-second) intervals for me.
Avoid over-cooking and burning the chocolate. Note: if you're familiar with using a double boiler, feel free to use this technique using a small saucepan instead of microwaving the chocolate.
Pour the melted chocolate on the prepared baking sheet.
Sprinkle the chopped pistachios over the melted chocolate. Gently press the pistachios into the melted chocolate.
If you'd like, you can make two different types of chocolate bark using various toppings.
Transfer the sheet pan to your refrigerator (if you have room) and refrigerate until the chocolate has completely set up, about 15 to 20 minutes. Break the chocolate into smaller hunks. Enjoy, or give as edible gifts!
If you don't have room in your refrigerator, leave the pistachio bark on the counter. It will solidify, but will take about 2 hours.