Place softened butter in a mixing bowl and beat on medium-high speed for about 1 minute until it becomes creamy and pale. Add the brown sugar and granulated sugar gradually, continuing to beat for another 2-3 minutes until the mixture is noticeably fluffy and lightened in color. This creaming process incorporates air into the dough, which creates the tender, delicate crumb characteristic of shortbread. I prefer to use a stand mixer for this step, but a hand mixer works just as well.

With the mixer still running on medium speed, add the room-temperature egg yolk and vanilla extract to the creamed butter mixture. Beat for about 1 minute until the mixture is smooth and fully combined. Room-temperature ingredients emulsify better with the butter, creating a more uniform dough and improved texture in the final cookies.

In a separate small bowl, whisk together the flour and salt to combine evenly and distribute the salt throughout. Reduce the mixer speed to low and add the flour mixture to the wet ingredients, mixing just until combined—stop as soon as you don't see dry flour streaks. Overmixing at this stage develops gluten, which makes the cookies tough rather than tender. Switch to a spatula and fold in the chopped dark chocolate pieces until evenly distributed throughout the dough.

Divide the dough in half and shape each portion into a log roughly 6-7 inches long and 1.5 inches in diameter. Wrap each log tightly in plastic wrap and refrigerate for at least 2 hours until very firm. Chilling is essential here—it prevents the dough from spreading too much during baking and allows the flavors to meld. I like to make these logs the day before baking for maximum convenience and better flavor development.

Preheat your oven to 350°F and line two baking sheets with parchment paper. Remove the plastic wrap from one dough log and roll it gently in granulated sugar to coat the exterior—this adds a pleasant crunch and sparkle. Using a sharp knife dipped in warm water (and wiped clean between cuts), slice the log into 1/2-inch thick rounds. Arrange the rounds on the prepared baking sheets about 1 inch apart to allow for slight spreading during baking.

Sprinkle each cookie round lightly with flaky sea salt—this enhances the chocolate flavor and adds a sophisticated contrast to the sweetness. Bake for 12-15 minutes until the edges are golden brown but the centers still appear slightly underdone; the cookies will continue to firm up as they cool on the baking sheet. I remove them when the edges just turn golden because residual heat finishes the cooking, resulting in a perfectly tender shortbread with a slightly crisp edge.