Melt your unsalted butter in the microwave or on the stovetop. Pour it into a medium mixing bowl and let it sit for a few minutes to cool down to room temperature so it doesn't cook the egg.
Add the brown sugar, white sugar, fine sea salt, and vanilla extract to the cooled butter. Whisk them together thoroughly until smooth.
Add the room-temperature egg yolk and whisk vigorously until the mixture looks glossy, thick, and well-combined.
Sift or pour the all-purpose flour and baking soda into the bowl. Use a rubber spatula to gently fold everything together. Stop mixing the exact moment you no longer see streaks of white flour.
Fold in your chopped chocolate chunks or chips until they are evenly distributed through the dough.
Scoop the dough into 8 equal balls using a cookie scoop and place them onto your prepared baking sheet. Bake for 9–11 minutes until the edges are golden brown, but the centers still look slightly soft.
Let the cookies sit on the hot baking sheet for 5 minutes to finish setting. Sprinkle with a little flaky sea salt if desired, then transfer to a wire rack or serve warm!
Notes
Use a Kitchen Scale: Weighing your flour is critical for this small batch. Cups easily pack in too much flour, which will accidentally turn these cookies dry and cakey instead of dense and chewy. For the perfect texture, measure in grams.
Use a Room-Temperature Yolk: Take your egg out of the fridge 15 minutes before baking. A cold yolk will seize up your melted butter and ruin the smooth dough emulsion.
Chop Your Own Chocolate: Instead of bags of chips, chop up a premium chocolate bar. This creates thin shards and uneven chunks that melt into gorgeous "chocolate pools."
Use a Cookie Scoop: This guarantees all 8 cookies are exactly the same size, ensuring they finish baking at the exact same moment.
Watch the Oven Clock: Small batches turn from perfect to overbaked in under 60 seconds. Pull them out when the edges look golden brown but the center is still soft to the touch.
The Pan Bang Trick: Halfway through baking, gently lift and drop the baking sheet against your oven rack. This flattens the center and creates beautiful, professional-looking bakery ripples.
The Cookie Scoot: If your cookies bake into wonky shapes, place a large circular glass or cookie cutter over them while hot and gently swirl it in a circular motion to shape them into perfect circles.