Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone mats. In a large mixing bowl, beat the softened butter, white sugar, and brown sugar together using an electric mixer on medium-high speed. Continue mixing for about 2-3 minutes until the mixture looks light and fluffy. Add the egg and vanilla extract, beating again until fully combined and smooth.
In a separate medium bowl, whisk together the flour, cocoa powder, baking soda, and salt to remove any lumps. Gradually add these dry ingredients to the butter mixture while mixing on low speed. Be careful not to overmix; stop as soon as you no longer see streaks of white flour. Use a sturdy spatula to fold in the chocolate chips by hand, ensuring they are evenly distributed throughout the dark dough.
Scoop rounded tablespoons of dough and place them on the prepared baking sheets, spacing them about 2 inches apart to allow for spreading. Bake for 9 to 11 minutes. The cookies will look slightly soft and underbaked in the center, but they will firm up as they cool. Let the cookies rest on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. This resting period ensures the perfect chewy texture.