In a large mixing bowl, combine the melted and cooled butter, brown sugar, and granulated sugar. Whisk until smooth and well combined.
Add the egg, egg yolk, and vanilla extract. Whisk until the mixture is smooth and completely combined.
Add the all-purpose flour, baking soda, and salt to the wet ingredients. Mix just until a cookie dough forms.
Gently fold in the chocolate chips or chocolate chunks until evenly distributed throughout the dough.
Cover the dough and refrigerate for 30–60 minutes.
Preheat the oven to 350°F and line a large baking sheet with parchment paper.
Bake at 350°F for 9–11 minutes.The edges should look set and lightly golden, while the centers may still look slightly soft or underdone. Don't wait until the entire cookie looks completely baked—the cookies will continue cooking as they cool on the hot baking sheet. Allow the cookies to cool on the baking sheet for 10 minutes before carefully transferring them to a wire cooling rack to cool completely.