Start by lining a baking sheet with parchment paper. This will make it easy to lift the bark off once it’s set.
In a double boiler, melt the chopped bittersweet chocolate over low heat. Stir continuously with a spatula until it becomes smooth and glossy. Be cautious not to let any water get into the chocolate.
Once melted, pour the bittersweet chocolate onto the prepared baking sheet. Use the spatula to spread it into an even layer, about ¼ inch thick. Place it in the fridge for about 15 minutes to set slightly.
While the bittersweet chocolate is setting, clean the double boiler and melt the chopped white chocolate in the same manner. Stir until smooth, then remove from heat.
Add the tablespoon of matcha powder to the melted white chocolate. Stir well until the matcha is fully incorporated and the mixture is a vibrant green color.
Retrieve the baking sheet from the fridge. Pour the matcha white chocolate over the set bittersweet chocolate, spreading it carefully to cover the surface evenly.
Sprinkle the chopped toasted pistachios generously over the top of the white chocolate layer. Press them down gently with your hands to ensure they stick.
Finish off by sprinkling the flaky sea salt over the entire surface. This will enhance the flavors and add a delightful crunch.
Return the baking sheet to the refrigerator and allow the bark to set completely, about 1 hour. Alternatively, you can let it set at room temperature, but this will take longer.
Once set, remove the bark from the fridge and lift it from the baking sheet using the edges of the parchment paper. Break it into pieces of your desired size, and it’s ready to serve!