Preheat oven to 325°. Line an 8x8x2 inches glass baking dish with foil, pressing firmly into pan and leaving a 2 inches overhang. Coat foil with nonstick spray; set baking dish aside.
Melt butter in a small sauce-pan over medium heat. Let cool slightly. Whisk sugar, cocoa, and salt in a medium bowl to combine. Pour butter in a steady stream into dry ingredients, whisking constantly to blend. Whisk in vanilla. Add eggs one at a time, beating vigorously to blend after each addition. Add flour and stir until just combined (do not overmix). Scrape 1/2 of the batter into prepared pan; smooth top.
Top with Oreo cookies. Top with the remaining brownie batter being sure to cover up the Oreos.
Bake until top begins to crack and a toothpick inserted into the center comes out with a few moist crumbs attached, 30-40 minutes.
Transfer pan to a wire rack; let cool completely in pan. Using foil overhang, lift brownie out of pan; transfer to a cutting board. Cut into 9 large squares or 12-16 smaller squares.