In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.

In a large mixing bowl, combine the slightly cooled melted butter, packed light brown sugar, and granulated sugar. Beat with an electric mixer on medium speed for 2-3 minutes until light and fluffy.

Add the whole egg, egg yolk, and vanilla extract to the butter mixture. Beat on medium speed for another 1-2 minutes until well combined and smooth, scraping down the sides of the bowl as needed.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; stop as soon as no streaks of flour remain.

Using a rubber spatula, fold in the semi-sweet and milk chocolate chips until evenly distributed throughout the dough.

Cover the bowl with plastic wrap and refrigerate the cookie dough for at least 30 minutes, or up to 1 hour. Chilling is crucial for chewy cookies and prevents spreading.

While the dough chills, preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper.

Scoop rounded tablespoons of cookie dough (about 1.5-2 tablespoons per cookie) onto the prepared baking sheets, leaving about 2 inches between each cookie. For a bakery-style look, press a few extra chocolate chips onto the tops of the scooped dough balls before baking. Sprinkle with flaky sea salt, if desired.

Bake one sheet at a time for 10-12 minutes, or until the edges are golden brown and the centers are still slightly soft and puffy.

Remove the baking sheets from the oven and let the cookies cool on the sheets for 5-10 minutes before transferring them to a wire rack to cool completely. This step is essential for the cookies to set and become perfectly chewy.

Repeat with the remaining cookie dough, ensuring baking sheets are cool or using fresh parchment paper for subsequent batches.