Preheat your oven to 350°F/175°C. Grease and line an 8×8 pan with parchment paper so the parchment slightly overhangs. This will make it easier to lift the brownies out once they've baked and cooled.
Make brownie layer
In a small bowl, whisk together the flour and salt.
In a large jug, whisk eggs, , sugars, oil and vanilla extract.
Over a double boiler, melt the butter and chocolate over low heat until it’s completely melted and smooth. Take it off the heat. You can also melt the butter and chocolate in the microwave in 20 second increments until melted and smooth.
Add the cocoa powder to the chocolate and whisk until smooth.
Pour the sugar mixture into the chocolate mixture and whisk until smooth.
Add the flour mixture and fold, making sure there are no flour pockets. Fold in Oreo crumbles. Set aside.
Make cookie layer
In a stand mixer fitted with the paddle attachment, or in a large bowl with an hand mixer, cream your butter until smooth.
Add both sugar and light brown sugar and beat on medium speed for 2-3 minutes until light and fluffy.
Add your egg and vanilla extract and beat once more.
In a small bowl, whisk together your flour, baking soda, salt and nutmeg.
Use a rubber spatula to fold in your dry ingredients into your wet ingredients.
Fold in your chocolate chips.
Dollop the brownie batter in the prepared pan, almost in a checkerboard pattern - or just make sure you leave space for the cookie dough. Once all the brownie batter is in, dollop the cookie dough on top and in the empty spaces. Use a butter knife to swirl them together, but don't overmix!
Bake for 25-30 minutes, the edges should be set and golden and the middle might still be underdone. It will continue baking once you take it out of the oven. Allow it to cool completely in the pan before removing and slicing.
Notes
Room Temperature: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients about 2 hours before you start baking.
Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.
Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.
Storage: keep in an airtight container for up to 4 days. You can freeze the bars for up to 3 months. Let them defrost in the fridge overnight before eating.