In a medium bowl, combine the flour, cocoa powder, salt and baking soda. In the bowl of a stand mixer fitted with a paddle attachment, or in a large bowl with a handheld mixer, beat the butter and brown sugar on high speed until light and creamy, about 3-4 minutes. Scrape down the bowl and add the vanilla, followed by the egg. Add the flour mixture and mix on low speed until incorporated. Stir in the chocolate chunks and the pistachios.

Divide the dough into 4 equal pieces. Roll each piece into a log about 8 inches long and about 2 inches wide. Wrap in plastic or parchment paper and place in the freezer for at least 4 hours and up to 2 months (cookies will be easiest to slice completely frozen).

Use a sharp knife to slice the frozen logs of cookie dough into cookies 1/4-inch thick. Place 1 inch apart on the prepared sheet pans.

Bake for 10-12 minutes, until cookies appear dry and set. Remove from oven and immediately sprinkle with the coarse salt. Let cool immediately.