Preheat oven to 375℉ and line two baking sheets with parchment paper.
In a medium bowl, whisk together flour, salt, baking soda, cornstarch, and baking powder. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment (or in a large mixing bowl with a hand-held mixer), cream the butter and sugars until light and fluffy, 2-3 minutes.
Add the egg, vanilla extract, and sourdough discard and mix on low until smooth and combined.
Add the flour mixture and mix on low until combined. Stir in the chocolate chips or chocolate chunks.
Using a cookie scoop or a small spoon, scoop cookie dough (about 2 Tablespoons per cookie) onto the prepared baking sheets, leaving room for the cookies to spread. Bake for 10-12 minutes until lightly golden. The cookies will look puffy when they come out of the oven, with the edges slightly browned. They will deflate some as they cool.
Optional: While the cookies are warm on the baking sheet, carefully place a sturdy round glass around the edges of the cookie and swirl slightly. This will loosen the cookie from the baking sheet and make the edges perfectly round. Keep in mind, you're not "trimming" the cookie, and you don't want to swirl too aggressively - you'll crush the cookie. Let the cookies cool for 2-3 minutes on the baking sheet. Transfer to a cooling rack to cool fully.
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Notes
Nutrition information is per cookie (1 serving = 1 cookie). Using a 2-tablespoon cookie scoop, this recipe makes about 32 cookies. This recipe was updated in April 2025.