In a medium bowl, whisk together the flour, cocoa powder, salt, and baking soda.
In a separate mixing bowl, beat the butter, brown sugar and vanilla extract with an electric mixer until light and fluffy (approximately 5 minutes).
Add in the dry ingredients to the butter mixture and mix until well combined.
Fold in the egg white, followed by the chopped chocolate and pistachios using a rubber spatula.
Push the dough together on a clean work surface and form it into two 8-inch logs. Wrap each log individually with parchment paper and refrigerate for at least 1 hour.
Preheat your oven to 350 degrees F (175 degrees C).
Using a sharp knife, cut the chilled dough logs into 1/2-inch thick rounds and place them onto a baking sheet lined with parchment paper.
Sprinkle each cookie round with a pinch of flaky sea salt.
Bake the cookies for approximately 9 minutes or until they are firm to the touch.
Let the cookies cool on a wire rack before serving.