In a large mixing bowl, add flour, cornstarch, baking soda, baking powder, and salt. Whisk until uniform.

In another large bowl, combine melted butter, brown sugar, and granulated sugar. Mix for 30 seconds until well blended.

Add the egg, extra yolk, and vanilla, mixing until thick, then add the sourdough discard. Whisk until the mixture is smooth.

Fold the dry ingredients into the wet mixture, mixing just until no dry streaks remain. Stir in the chocolate chips until evenly dispersed. Don’t over-mix.

Cover the dough and chill in the refrigerator for 4 to 48 hours (the longer, the deeper the flavor).

Preheat the oven to 350°F (175°C). For larger cookies, scoop 65-70 g balls onto parchment-lined baking sheets, spacing them about 3 inches apart. For smaller cookies, scoop 45 g balls. Roll the dough into balls using your hands.

Bake one cookie sheet at a time for 12-15 minutes for larger cookies and 10-12 minutes for smaller cookies.

The rims should be golden brown, and the centers should look soft but no longer shiny. Don’t over-bake; the cookies will continue to set outside the oven.

Repeat this with the second baking sheet.

For a bakery-style look, push some extra chocolate chips into the tops of the cookies right after taking them out of the oven.

Let the cookies cool for 5 minutes on the baking sheet.

After that, transfer them (with the parchment paper) to a rack to finish cooling.