Melt the butter in the microwave in 30-second bursts until just melted, then pour it into the mixing bowl and let it cool for approximately 5 minutes. It should be warm but not hot.
Add the white and brown sugars to the bowl with the butter, and whisk gently just until combined.
Add the egg, egg yolk, and vanilla, and whisk just until smooth and combined. Don't try to incorporate any air or mix too vigorously.
Sift the dry ingredients, including flour, cornstarch, baking soda, and salt. Switch to a wooden spoon or rubber spatula and mix just until combined into a soft dough. Be careful not to overmix.
Add the chocolate chips and mix until evenly dispersed.
Use a small cookie scoop or spoon to portion out balls of cookie dough, approximately 2 inches in diameter. Place them on a cookie sheet - there is no need to space them out right now as we will move them to another cookie sheet for baking. Cover with plastic wrap, then place them into the fridge to chill for at least an hour. For even better flavor and texture, let them chill overnight.
When you are ready to bake, preheat the oven to 350℉ / 180℃ (160℃ fan-assisted) and line another baking sheet with parchment paper. Lay some of the cookie dough balls out on the sheet, with enough room to spread.
Bake for 12 minutes, or until the edges of the cookie turn golden brown and the tops of the cookies are dry. The centers may still look pale and underbaked, but they will continue to bake on the pan as they cool.
Remove the cookies from the oven, and let them sit on the baking sheet for 10 minutes before using a cookie lifter to transfer them to a wire rack to cool. They may look slightly risen and puffy while coming out of the oven, but they will settle down as they cool and flatten out into that gorgeous "crinkly" look.
Notes
Do you have to chill cookie dough?As these are melted butter chocolate chip cookies, it is essential to chill the cookie dough for at least an hour before baking. Letting the cookies chill for 8-24 hours will give even better results, but you can just chill them for an hour if needed. See the post above for more info on this. Make Ahead Instructions These cookies are great for making ahead! You can keep the scooped cookie dough balls in the fridge for 3 days or in the freezer for up to 3 months. I love keeping a freezer bag of frozen cookie dough balls that are ready to go at a moment's notice. Whenever we have unexpected guests I just pop a few in the oven and have warm chocolate chip cookies with no effort. You can bake them from frozen, you may need to add an extra minute or two onto the bake time, but still take them out once the edges start to turn golden brown.