Preheat the oven to 325 degrees Fahrenheit. In a bottom of a 9x9 baking dish, and parchment paper and grease. set aside. (these cookies are very thick and gooey, this can be made in a 13x9 baking dish if you prefer thinner brownies, baking time will have to be reduced)
In a large bowl, beat the melted butter and sugar until smooth. Add the eggs and vanilla, and whisk to combine. Stir in the cocoa powder, mixing just until smooth.
1 cup unsalted butter melted, 1 3/4 cups granulated sugar, 4 large eggs, 2 teaspoons vanilla extract, 1 cup unsweetened cocoa powder
Using a rubber spatula, stir in the flour, cornstarch, baking soda, and salt just until combined and no clumps remain. Gently fold in the chocolate chunks
1 cup all-purpose flour, 4 teaspoons cornstarch, 1/2 teaspoon baking soda, 3/4 teaspoon sea salt, 1 8 ounce semi-sweet chocolate bar, cut into chunks
Pour the batter into the prepared baking dish, spreading it out evenly. Top with Oreo cookie chunks.
1 cup Oreo cookie chunks
Bake 35-40 minutes or until the batter is set and a cake tester can be inserted into the center and removed with just a few moist crumbs attached. Be careful not to overcook!
Set aside to cool completely.
Store unsliced in an airtight container at room temperature for up to 4 days or sliced for 1-2 days.