Brown the butter: Place the cubed unsalted butter in a light-colored, medium saucepan over medium heat. Stir frequently as the butter foams, then browns and smells nutty (about 6–8 minutes). Watch closely — butter can go from browned to burnt fast. When you see browned milk solids at the bottom and a deep nutty aroma, remove from heat and transfer to a heatproof bowl to stop cooking. Cooling concentrates flavor and prevents the butter from scrambling the eggs when mixed.

Chill the butter: Let the browned butter cool until it is just warm (about 20–30 minutes). You want it liquid but not hot — warm butter carries the toasted flavor and helps dissolve sugar; hot butter will cook the eggs and ruin texture.

Preheat oven and prepare pans: Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats. Using insulated sheets or double-sheeting prevents the bottoms from overbrowning, giving you a crisp edge and tender interior.

Combine dry ingredients: Whisk together the all-purpose flour, baking soda, and 1 teaspoon fine sea salt in a bowl. Whisking evenly distributes the leavening and salt so cookies rise and season uniformly.

Mix sugars and brown butter: In a large bowl, whisk the brown (cooled) butter with the granulated and brown sugars until smooth and glossy. The liquid brown butter will look syrupy. This step dissolves some sugar and begins building the cookie's chew and caramel notes — the brown butter's flavor infuses the sugars.

Add eggs and vanilla: Add the whole egg and extra yolk one at a time, whisking until fully combined; then stir in the vanilla. Eggs provide structure and moisture; the extra yolk increases richness and chewiness without making the dough cakey.

Fold in dry ingredients: Add the flour mixture to the wet mixture and fold with a spatula just until you no longer see streaks of flour. Overmixing develops gluten and yields tough cookies — stop when combined to keep centers tender.

Add chocolate: Fold in the chocolate chips or chopped chocolate so every bite has pockets of melty chocolate. If using chopped chocolate, you'll get varied pockets of melt and a more gourmet texture.

Chill the dough (important): Cover the bowl and chill the dough for at least 30 minutes, up to 24 hours. Chilling firms the fat, allowing the cookies to spread less and develop deeper flavor as the dry ingredients hydrate. If short on time, 30 minutes still helps; longer (overnight) yields the best texture and flavor.

Portion the dough: Using a 1.5-tablespoon scoop (or tablespoon for slightly smaller cookies), portion dough into uniform balls about 24 total. Place them 2 inches apart on the prepared sheets. Uniform size yields even baking; crowded dough will steam and not brown properly.

Bake: Bake one sheet at a time in the center of the preheated oven for 12–14 minutes, rotating halfway through. Remove when edges are golden-brown and centers still look slightly soft — they’ll continue to set on the hot sheet. This gives crisp edges and chewy centers.

Finish with flaky salt and cool: Immediately after removing from the oven, sprinkle each cookie with a pinch of flaky sea salt. Let the cookies rest on the baking sheet for 5–7 minutes, then transfer to a wire rack to cool completely. The short rest lets cookies finish cooking gently and prevents them from breaking apart when moved.

Store or serve: Store cooled cookies in an airtight container at room temperature for up to 4 days, or freeze for up to 3 months. Rewarm briefly in a 300°F (150°C) oven for a fresh-baked feel.