Preheat oven to 350°F. Create a parchment paper sling to line the bottom of a 9x9-inch square baking pan.
In a large bowl, beat the butter, brown sugar, and granulated sugar together for 2 minutes until light. Add the egg and vanilla, mixing well and scraping down the sides of the bowl as needed.
1/2 cup salted butter, 1/2 cup brown sugar, 1/4 cup granulated sugar, 1 large egg, 1 teaspoon vanilla extract
Add the flour, baking soda, and salt, stirring just until combined. Stir in chocolate chips, then set press into the prepared pan.
1 1/4 cups all-purpose flour, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 1 cup semisweet chocolate chips
Arrange the Oreos on top of the cookie dough in a single layer. You may need to break some in half to fill significant gaps.
20-22 double stuffed Oreos
In another bowl, whisk the melted butter, granulated sugar, brown sugar, and cocoa powder together. Add eggs, vanilla, and salt, whisking until combined.
1/2 cup salted butter, 3/4 cups granulated sugar, 1/4 cup brown sugar, 1/2 cup unsweetened cocoa powder, 2 large eggs, 1 teaspoon vanilla extract, 1/2 teaspoon salt
Add the flour to the chocolate mixture, stirring just until incorporated, then spread this over the Oreo layer.
1/2 cup all-purpose flour
Bake for 35-40 minutes until a toothpick inserted in the center comes out clean with just a few crumbs.
Cool completely (at least 2 hours) before removing the brownies from the pan and cutting into squares and serving.