1Add 1 cup of unsalted butter to a bowl and microwave in 30 second increments until melted.
2Add 1 cup of granulated sugar and 1 cup of brown sugar. Whisk together.
3Add in 2 large eggs and 1 egg yolk, 2 teaspoons of vanilla and ¾ cup of sourdough discard. Mix and set aside.
4In another bowl, add 3 cups of all-purpose flour, 1 teaspoon of baking soda, ½ teaspoon of baking powder, 1 ½ teaspoons of salt and 2 teaspoons of cornstarch. Mix well.
5Add the dry ingredients to the wet ingredients and mix until just combined.
6Fold in 1 ½ cups of chocolate chips.
7Pour the mixture into a greased 9x13 baking dish and spread evenly.
8Bake at 350 degrees for 40-45 minutes, or until a toothpick comes out clean when inserted into the center.