In a mixing bowl, whisk flour and fine salt, then sift in the cocoa powder. Whisk to combine.
In the bowl of a standing mixer, cream together butter and sugar, scraping down the sides of the bowl with a rubber spatula, until light and fluffy, about 1 minute. Add the sour cream and mix for a few seconds to combine. On the lowest speed, gradually add the cocoa mixture, mixing until the dough comes together. Mix in the chopped chocolate and pistachios. Once the dough becomes slightly tacky and can easily be molded into a ball, dump the dough onto a work surface.
Using your hands, mound the dough into a big ball. Cut the ball in half and roll each half into a 10-inch-long log. Wrap each log tightly in wax paper, twisting the ends, so you have 2 logs of dough. Refrigerate the wrapped dough for at least 1 hour or freeze for later use.
When ready to bake, preheat oven to 350˚F and remove the logs of dough from the refrigerator to soften a bit. Line 2 cookie sheets with parchment paper.
Unwrap the cookie dough and, using a sharp knife, slice the dough into ½-inch-thick rounds and arrange on the prepared cookies sheets about 2 inches apart.
Bake for 12 to 15 minutes or until they start to brown and look dry on top. Remove from the oven and sprinkle with a touch of sea salt. Let cool completely before eating.
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