Preheat oven according to brownie package directions. If using 2 packages in a 9 X 13 pan, brownies will need to stay in for about 45-55 minutes. In a large bowl, combine 2 packages of brownie mix and 4 tablespoons flour. Using a whisk, stir to combine.
In a medium bowl, combine wet ingredients. For this recipe, the brownie mix package asked for eggs, water, oil. I substituted the oil with melted butter. Whisk wet ingredients and add 2 teaspoons vanilla extract (optional).
Pour wet ingredients into dry ingredients and mix until combined; set aside.
Grease the bottom and sides of an 9 X 13 inch pan with room temperature butter. Pour a little more than 1/2 the brownie batter into the bottom of greased pan. Cover with oreos across the top of brownie batter. Gently push each oreo down. Top oreos with remaining brownie batter. Cover with foil and bake in oven. Insert a clean, dry knife into the center of brownie pan. When the knife comes out clean, brownies can be taken out of oven. Crumble any left over oreas over top or place large chunks into the top of brownies before they cool. Allow to cool before slicing.