1 C plus 2 tbsp / 255 g total salted butter, cold (room temperature if you're using a handheld mixer), cut into ½-inch pieces
½ C / 101 g granulated sugar
¼ C / 55 g light brown sugar
1 tsp vanilla extract
2½ C / 326 g all-purpose flour (see note)
6 oz / 170 g semi-sweet or bittersweet dark chocolate, chopped into chunks
1 large egg, beaten
Demerara sugar, for rolling
Flaky sea salt, for sprinkling
Line two rimmed baking sheets with parchment paper.
Using mixer beat the butter, both sugars, and vanilla on medium-high till it’s super light and fluffy (3 to 5 minutes for a stand mixer; 6 to 8 for a hand mixer).
Scrape down the sides of the bowl and, with the mixer on low, slowly add flour
Add chocolate chunks, and mix just to blend.
At this point, the dough should be smooth and feel like Play-Doh with no pockets of flour.
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
Don't be afraid to make them compact. Shortbread is supposed to be dense.
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 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
If the cookies break or fall apart, just press them back together
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.
Let cool slightly before eating them all.