In a bowl, beat the room temperature butter until light and fluffy using an electric whisk or stand mixer fitted with a paddle attachment.
Add the cane or castor sugar & light brown sugar and beat until creamy. Mix in the room temperature milk in two parts & vanilla extract until combined, then add the cocoa powder and mix well.
Add the flour, cornstarch, baking soda, and salt, mixing just until combined. Fold in the chopped walnuts and chocolate chips, reserving a few to press on top before baking.
Scoop the dough into balls (about 135g each for large NYC-style cookies or 65g for smaller ones). Cover and Chill the scooped dough for 1 hour.
Preheat the oven to 350°F (175°C)and line a baking tray with parchment paper. Place cookie dough on the prepared tray, leaving space between each cookie, and press extra chocolate chips and walnuts on top if desired.
Bake for about 15-16 minutes, or 9-10minutes for smaller cookies until the edges are set while the centers remain soft. For perfectly round cookies, gently reshape them with a cookie cutter or a bowl right after baking while still warm.
Sprinkle with flaky sea salt if desired and let cool on the tray for 30 minutes before transferring to a wire rack.
Store the cookies in an airtight container at room temperature for up to three days. For a freshly baked feel, warm a cookie in the microwave for 10 to 15 seconds to bring back the gooey center. The dough can also be frozen and baked fresh when needed.