Line a baking sheet with parchment paper.

Melt the dark chocolate in a microwave-safe bowl in 30-second intervals in a microwave oven, stirring between each interval. Stop microwaving once 80% of the dark chocolate is melted. Stir until all of the dark chocolate is melted. My microwave oven melted the dark chocolate in 2:30 minutes.

Stir half the chopped pistachios (1 cup) into the melted dark chocolate.

Spread melted dark chocolate with pistachios on parchment-lined baking sheet in an even layer about ⅛" thick, shaping sides into a rectangle if you can.

Divide the white chocolate into two microwave safe bowls. Melt white chocolate in both bowls in the same way as the dark chocolate, in 30-second intervals in a microwave oven, stirring between each interval. Stop microwaving once 80% of the white chocolate is melted. Stir until all of the white chocolate is melted. My microwave oven melted the white chocolate in 2:00 minutes (four 30-second intervals).

Sift the matcha powder into one of the bowls of melted white chocolate, then stir well until the white chocolate is a uniform green color.

Alternately drop large spoonfuls of the melted white chocolate and matcha chocolate in a random pattern over the dark chocolate. Using a toothpick, and without touching too much of the dark chocolate underneath, lightly swirl the white and matcha chocolates into a marble pattern. Scatter the remaining pistachios on top, and lightly press into the chocolate. Sprinkle with flaky seat salt if using.

Let chocolate cool completely for about 2 hours at room temperature (or about a half hour if you can fit your baking sheet into the refrigerator). Gently break or cut into pieces.

Pistachio Matcha Dark Chocolate Bark will keep for about a week stored in a cool place in an airtight container between layers of parchment. Do not refrigerate.