Set your oven to 400°F and position a rack in the center. While the oven heats, prepare your baking sheets. This is important: use light-colored baking sheets, not dark ones. Dark sheets conduct heat too aggressively and can over-brown the bottoms of these thick cookies before the centers bake through. If you only have dark sheets, try placing a second light-colored sheet underneath as an insulator.

In a large mixing bowl, combine your cold butter cubes with both sugars. Using an electric mixer on medium-high speed, beat for 3 to 4 minutes until the mixture becomes light, fluffy, and noticeably pale. You're aerating the butter, which creates lift and a tender crumb structure. About halfway through this mixing time, pause and scrape down the sides of the bowl with a rubber spatula. This ensures every bit of butter gets incorporated evenly, preventing any hard butter pieces that could create tough streaks in your cookies.

Crack your eggs directly into the creamed mixture and add the vanilla extract if you're using it. Mix on medium speed for about 1 minute until everything looks smooth and creamy. The mixture should be homogeneous and smooth—no visible egg white streaks or lumps of undissolved butter. This step matters because eggs act as both a binder and a tenderizer. Mixing them in thoroughly creates structure that supports those large, heavy chocolate chips without the cookie spreading too thin during baking.

In a separate bowl, whisk together your flour, unsweetened cocoa powder, cornstarch, baking soda, and salt. Whisking for about 30 seconds aerates the dry ingredients and distributes the leavening agent evenly. This prevents pockets of baking soda that could create an unpleasant soapy flavor. The cornstarch is doing important work here. It reduces the gluten network in the flour, which is exactly what you want for a tender, chewy cookie rather than a tough, dense one.

Lower your mixer to low speed and add the dry ingredient mixture to the butter-egg mixture. Fold gently for about 30 seconds to 1 minute, just until combined. You want a few flour streaks visible at the end—this is intentional. Overmixing develops gluten, which creates tough cookies instead of tender ones. The dough should look shaggy and textured at this point, not smooth and uniform. That's exactly right.

Add your white chocolate chips and milk chocolate chips to the dough and fold gently by hand using a rubber spatula. Mix just until the chips are distributed throughout, about 30 to 45 seconds. A few streaks of undistributed dough are fine—you want to avoid overworking this dough.

Here's where a food scale becomes your best friend. I recommend using one because it guarantees perfectly even baking—no guessing about whether your cookies are the right size. Divide your dough into 12 equal portions of 6 ounces each. Roll each portion gently into a smooth ball and place on your prepared light-colored baking sheets, leaving about 3 inches between each cookie for spreading. These cookies will spread during baking, so proper spacing is important. If your baking sheet feels crowded, use two sheets instead of forcing all 12 cookies onto one.

Place your baking sheets in the preheated 400°F oven and bake for 9 to 11 minutes. You're looking for the edges to look set and slightly darker while the center still appears soft and slightly underdone. The cookies will continue cooking from residual heat after they come out of the oven. At 9 minutes, check one cookie by pressing gently in the center—it should feel barely firm, with a little give. If the centers still feel soupy, check again in 1-minute intervals. Overbaking by even 2 minutes creates dry, cakey cookies instead of chewy ones.

Remove the baking sheets from the oven and let the cookies rest on the hot pan for at least 10 minutes without moving them. This is critical. During this resting period, the structure sets up completely and the cookies develop their signature chewiness. The cookies will feel soft and pliable when first removed from the oven, but they'll firm up beautifully as they cool. After 10 minutes, you can gently transfer them to a wire cooling rack if you'd like, but honestly, they're perfect eaten warm right off the pan with a cold glass of milk.