Salted Butter Chocolate Chunk Shortbread Cookies 5 Steps
Line two rimmed baking sheets with parchment paper. Using a stand mixer fitted with the paddle attachment or an electric hand mixer, beat the butter, both sugars and vanilla on medium-high until it’s super light and fluffy (3 to 5 minutes for a stand mixer; 6 to 8 for a hand mixer). Using a spatula, scrape down the sides of the bowl and, with the mixer on low, slowly add the flour, followed by the chocolate chunks and mix just to blend. If necessary, knead the dough with your hands to make sure the flour is totally incorporated. The dough should be smooth.
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. (Don't be afraid to make them compact. Shortbread is supposed to be dense. That's part of why it's so good.) You can also do this using parchment paper, if you prefer, but plastic wrap is easier when it comes to shaping the log. Each half should form a 6-inch log, 2 to 2 1/4 inches in diameter. Chill until totally firm, about 2 hours.
Heat the oven to 350°F. 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 1/2-inch-thick rounds (if you hit a chocolate chunk, slowly saw back and forth through the chocolate). If the cookies break or fall apart, just press them back together — the dough is very forgiving. A few usually crack but don't be afraid. Place them on the prepared baking sheets about 1 inch apart (they won’t spread much). Sprinkle with flaky salt. Bake until the edges are just beginning to brown, 12 to 15 minutes.
You can shape these cookies using a straight edge life a knife. When they are still hot you can press the sides so they form 'neater' circles. Let cool slightly.