Mix the butter and sugars. In a large mixing bowl, combine the melted butter with the granulated sugar and brown sugar. Using an electric hand mixer, a stand mixer, or a spatula, mix until the sugars are fully incorporated and the mixture is smooth and glossy. Do not expect it to become light and fluffy, as melted butter won't trap air the way softened butter does.
Add the wet ingredients. Add the beaten egg and vanilla extract to the butter-sugar mixture. Mix with an electric hand mixer, a stand mixer, or a spatula until the egg is fully incorporated and the mixture is smooth, glossy, and well combined.
Combine the dry ingredients. In a separate medium bowl, whisk together the flour, baking soda, and salt until evenly combined. This ensures the leavening agent and salt are evenly distributed throughout the flour before they're added to the wet ingredients.
Gradually add the dry ingredients to the wet ingredients, gently mixing with a spatula or whisk until the dough starts to come together. When a few streaks of flour still remain, fold in the chocolate chunks. Continue mixing just until the flour is fully incorporated and the chocolate is evenly distributed throughout the dough. Be careful not to overmix.
Scoop the cookie dough. Using a medium cookie scoop or about 2 tablespoons or (30g) of dough, portion the dough into evenly sized balls. Place them on the prepared baking tray, leaving 2–3 inches (5–7 cm) of space between each cookie to allow for spreading. If desired, press a few extra chocolate chunks onto the tops for a bakery-style finish.
Bake the cookies. Bake in a preheated 350°F (175°C) oven for 9–10 minutes, or until the edges are lightly golden and the centres still look slightly underbaked. They will continue to set as they cool, giving you perfectly soft and chewy cookies.