Preheat oven to 350°F. Line a 9x13-inch pan with foil and spray with nonstick cooking spray.
Place chocolate and butter in a large microwave-safe bowl. Heat on high power in 30 second increments, stirring between each, until chocolate and butter are melted and smooth (about 2 minutes, depending on your microwave).
Stir sugar and cocoa into chocolate and butter mixture using a spatula or wooden spoon. Stir in eggs and vanilla, then stir in salt. Mixture will be grainy looking but smooth. Stir in flour carefully.
Pour 1/2 of batter into prepared pan. Top with Oreos evenly, then spread the remaining batter in the pan.
Bake for 25-35 minutes, or until the top is no longer glossy and a toothpick comes out with just a few crumbs when placed 2-inches from the side of the pan. Cool completely before slicing. I prefer mine baked only 25 minutes; if you want yours less fudgy, bake longer.