Step 1: Whisking the Dry Ingredients
In a medium-sized mixing bowl, thoroughly whisk together the all-purpose flour, cornstarch, baking soda, and kosher salt. Whisking ensures that the leavening agents and the crucial cornstarch are evenly distributed, which prevents uneven baking and ensures every single bite has the same tender texture.Step 2: Combining the Wet Ingredients
In a large mixing bowl, whisk the cooled melted butter, dark brown sugar, and granulated sugar until no lumps remain and the mixture is smooth. Actionable Tip: Vigorously whisking the sugars into the melted butter for about 1-2 minutes helps dissolve the sugar crystals, which leads to a shinier, more crinkly top crust on your finished cookies.Step 3: Emulsifying the Eggs and Vanilla
Add the room-temperature egg, the extra egg yolk, and the pure vanilla extract to the sugar-butter mixture. Whisk until the batter is completely smooth, glossy, and falls off the whisk in ribbons. The room temperature aspect is vital here; cold eggs will solidify the melted butter and break the emulsion.Step 4: Folding the Dough and Adding Mix-ins
Using a rubber silicone spatula, gently fold the dry flour mixture into the wet ingredients. Stop mixing when there are still a few visible streaks of flour. Overmixing develops gluten, which is the enemy of a soft center. Gently fold in your hand-chopped semi-sweet chocolate chunks until just combined.Step 5: The Mandatory Dough Chill
Cover the mixing bowl tightly with plastic wrap and place it in the refrigerator for at least 2 hours (or up to 48 hours for incredible flavor development). Pro Tip: As the dough rests, the flour absorbs the moisture from the eggs and butter. This process, known as hydration, results in a thicker dough that bakes up with a significantly softer, more concentrated core.Step 6: Baking and Intentional Underbaking
Preheat your oven to 325°F (165°C)—a slightly lower temperature that prevents the edges from burning before the center warms. Scoop 2-tablespoon-sized balls of dough onto a parchment-lined baking sheet, spacing them 2 inches apart. Bake for 9 to 11 minutes. Crucial step: Remove them from the oven while the centers still look incredibly pale, puffy, and slightly raw. They will continue to bake on the hot baking sheet for 10 minutes outside of the oven, resulting in the perfect soft center. Transfer to a wire cooling rack.
