In a medium mixing bowl, whisk together the flour, baking powder, cornstarch and salt. Set aside.
175 grams all-purpose flour, 4 grams baking powder, 5 grams cornstarch, 1 grams salt
To a bowl of a stand mixer with the paddle attachment, beat the butter and sugar on medium speed until lighter in color and fluffy. (about 3-4 minutes)
113 grams unsalted butter, softened, 150 grams sugar
While mixing on low, add the room-temperature sourdough discard, egg yolk, and vanilla extract and mix until combined.
100 grams sourdough discard, room temperature, 1 egg yolk, room temperature, 4 grams vanilla extract
Scrape down the sides and bottom of the bowl before adding the flour mixture and mixing on low until the flour is just combined.
Grease a 12 inch pizza pan (without the crisper holes) with cooking spray and shape the cookie dough into a 9-10 inch circle, about ¼ inch thick. Cover with plastic wrap and place the dough into the freezer for 15 minutes. Chilling the dough prevents the cookie from over spreading while baking. See Alternative Baking Options BelowLong Ferment: Instead of placing the shaped dough in the freezer, store in the fridge overnight and bake the following day. When ready to bake, preheat the oven to 350 degrees F.
Bake the sourdough cookie for 17-19 minutes or until the edges are just lightly golden brown. Be sure not to over bake. While the cookie is still hot, use the back of a spoon to smooth the edges of the cookie. Leave the cookie to cool completely on the cookie sheet.