Directions

• Line two rimmed baking sheets with parchment paper. Beat the butter, both sugars, and vanilla on medium-high till it’s very light and fluffy (3 to 5 minutes). Scrape down the sides of the bowl and slowly add the flour/salt mix, then the chocolate chunks, and mix just to blend. If necessary, knead the dough with your hands to make sure the flour is totally incorporated.
• Divide the dough in half, placing each half on a large piece of plastic wrap. Fold the plastic over so that it covers the dough to protect your hands from getting all sticky. Using your hands, form the dough into a log shape; rolling it on the counter will help you smooth it out, but don’t worry about getting it totally perfect. Each half should form a 6-inch log, 2 to 2¼-inches in diameter. Chill until totally firm, about 2 hours.
• Heat the oven to 350 degrees. Brush the outside of the logs with the beaten egg and roll them in the demerara sugar.
• Using a serrated knife, carefully slice each log into ½-inch-thick rounds. Place them on the prepared baking sheets about 1 inch apart. Sprinkle with flaky salt. Bake until the edges are just beginning to brown, 12 to 15 minutes.