To make the cookie dough, sift 4 cups of flour and salt into the bowl of an electric mixer. Add the butter and beat on low for 2 minutes until blended. In a small bowl, whisk together the sour cream and 2 egg yolks, then add this to the flour mixture and beat on medium for 1 minute. Use a spatula to finish mixing the dough until thoroughly combined if necessary. On a dry cutting board, divide the dough into four equal portions, wrap each one in plastic wrap and refrigerate for 1 hour.
To make the filling, preheat the oven to 350 degrees. Toast the pistachios on a baking sheet for 10 minutes. Cool them to room temperature. Place the chocolate, sugar, pistachios and cinnamon in the bowl of a food processor and pulse until finely chopped, about 15 seconds. Set aside. To assemble cookies, remove 1 piece of dough from the refrigerator. Place the dough on a clean, dry, well-floured work surface. Roll the dough into a rectangle 12 inches long and 8 inches wide. Trim the edges to form an even rectangle. Sprinkle ¼ of the filling mixture (about ¾ cup) over the entire surface of the dough. Using a pizza cutter, make 5 cuts across the width of the dough at 2-inch intervals, then diagonally cut each strip in half lengthwise from corner to corner. Roll each triangle from the base to the point and place on a cookie sheet. In a small bowl, whisk together the remaining egg yolk and water. Lightly brush each cookie with a pastry brush and then sprinkle on sugar. Bake for 24 minutes in the 350-degree oven.