Combine dry ingredients: In a large mixing bowl, add flour, cornstarch, baking soda, baking powder, and fine sea salt. Whisk until uniform.
Whisk wet ingredients: In another large mixing bowl, combine butter, brown sugar, granulated sugar, and whisk for about 30 seconds until well blended. Add egg, extra yolk, and vanilla extract. Continue whisking until thick and blended, then add sourdough discard and whisk until smooth.
Mix the dough: Fold the dry ingredients into the wet mixture, mixing just until streaks disappear. Stir in chocolate chips just until distributed, avoiding over-mixing.
Chill the dough: Cover and refrigerate for 4 to 48 hours to enhance flavor.
Scoop: Preheat the oven to 350°F/175°C. For larger cookies, scoop 65-70 g balls; for smaller, scoop 45 g balls, placing them on parchment-lined baking sheets.
Bake: Bake for 12 to 15 minutes for larger cookies and 10 to 12 for smaller cookies until rims are golden and centers are soft.
Make them beautiful: Push additional chocolate chips into the tops right after baking for a bakery-style appearance.
Cool: Let the cookies cool for 5 minutes on the baking sheet, then transfer to a cooling rack.