In a medium sauce pan, melt butter of medium heat. Stir in cocoa powder and vanilla. Pour mixture into a large mixing bowl. Add eggs, sugars, and flour and mix until all ingredients are incorporated and well combined. Pour half of batter into prepared pan. Lay a single layer of oreos side by side on top of batter. Pour remaining batter over oreos and spread evenly. Bake 30-40 minutes until a toothpick comes out mostly clean.