📖 Instructions

1. Before you even start, take your unsalted butter out of the fridge. You want it softened but not melted. If you press it lightly with your finger, it should make an indentation. Also, preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper or silicone mats. This is super important to prevent sticking and ensure even baking.
2. In a large mixing bowl, using an electric mixer (handheld or stand mixer with paddle attachment), cream together the softened unsalted butter, granulated sugar, and light brown sugar on medium speed. Beat it for about 2-3 minutes until the mixture is really light, fluffy, and looks pale. Make sure to scrape down the sides of the bowl occasionally with a rubber spatula to get everything incorporated.
3. Crack the eggs into a separate small bowl, one at a time, to make sure there are no shells. Add one large egg and the egg yolk to the creamed butter mixture. Beat on medium speed until well combined. Stir in the vanilla extract until it's just incorporated. Don't overmix here.
4. In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt. Give it a good whisk to ensure the baking soda and salt are evenly distributed throughout the flour.
5. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; overmixing can lead to tough cookies. Stop as soon as you see no more streaks of flour.
6. Divide the cookie dough into two equal portions. You can use a kitchen scale for accuracy if you're a stickler for perfection, or just eyeball it. Place each portion into a separate medium bowl.
7. To one portion of the dough, add the unsweetened cocoa powder. Mix it in until just combined and uniformly chocolatey. Don't go crazy mixing; gentle is the key here. You now have your vanilla dough and your chocolate dough.
8. Now for the fun part: marbling! Take a small spoonful of vanilla dough and place it on a clean surface. Then take a small spoonful of chocolate dough and place it right next to or on top of the vanilla dough. Gently press them together. You can repeat this a few times to get alternating colors. Then, with your hands, very gently roll the combined dough into a ball about 1.5 to 2 inches in diameter. As you roll, the colors will start to swirl naturally. You don't want to completely blend them, just a few swirls. It's okay if it's not perfect; imperfect swirls are beautiful too!
9. Place the marbled cookie dough balls onto the prepared baking sheets, leaving about 2 inches between each cookie to allow for spreading. You'll likely fit 12 cookies per sheet.
10. Bake one sheet at a time in the preheated oven for 9-11 minutes. The edges should be set and lightly golden, but the centers should still look slightly soft and gooey. This is the secret to that chewy texture! Don't be tempted to bake longer. If you have any doubts, err on the side of underbaking slightly.
11. Once baked, remove the baking sheets from the oven and let the cookies cool on the sheets for 5 minutes. They will continue to set during this time. After 5 minutes, carefully transfer the cookies to a wire rack to cool completely. This helps them firm up and keeps that lovely chewy texture. Enjoy them warm or at room temperature!