Preheat oven to 325°F. Spray a 9x9 pan with non-stick baking spray. Line the pan bottom and sides with parchment paper and spray again.
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Sift flour, cocoa powder, cornstarch, and salt in a basin. Set aside. Beat eggs, egg yolk, and sugar on high for 5 minutes in a mixer with a whisk attachment. Light, pale sugar is ideal.
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Melt butter in the microwave while mixing. It should cool somewhat as the eggs beat. Mix melted butter, oil, and vanilla on low until mixed. Add dry ingredients and mix on low until mixed.
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Scrape the bowl with a rubber spatula after stopping the mixer. Mix Oreos with chocolate chips. Pour batter into pan. Top with extra Oreos.Bake 45-55 minutes. Set boundaries and underdo the middle.
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If toothpick reveals batter, bake 5-10 minutes more. A toothpick with crumbs indicates readiness. Let it cool completely in the pan on a rack. Serve after cooling from pan.
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