In a small mixing bowl, cream the softened butter, brown sugar, and granulated sugar using a hand mixer or sturdy whisk for 2-3 minutes. The mixture should look light and fluffy. Proper creaming creates air pockets that result in a chewier texture and even spread.
Beat in the egg yolk and vanilla extract until fully combined, about 30 seconds. Scrape the bowl. Look for a glossy, cohesive texture. This builds emulsion, ensuring that the fat disperses and locks in moisture.
Add the flour, baking soda, and salt. Mix on low or fold with a rubber spatula until just combined and no dry streaks remain. Avoid overmixing, which can toughen the cookies, the dough should be thick and sticky but not wet.
Gently fold in the chocolate chips until evenly distributed. Use the spatula to turn the dough over itself rather than stirring aggressively. This ensures pockets of chocolate throughout, without causing the dough to seize or overwork.
Let the dough rest at room temperature for 10 minutes. This short rest allows the flour to hydrate fully and the dough to firm up, preventing overspreading and leading to thicker cookies with soft centers.
Preheat your oven to 375°F (190°C). Line a small baking sheet with parchment. Use a small scoop or tablespoon to portion the dough into 6 equal balls, spacing them 2 inches apart. Bake for 9-11 minutes, until edges are golden and centers look set but slightly underdone. Rotate the pan after 6 minutes for even baking.
Cool cookies on the baking sheet for 3 minutes, then transfer to a wire rack. This resting period lets the centers finish setting without overbaking the edges. Enjoy warm for gooey chocolate or let fully cool for ultimate chew.