Yield:24 brownies

For the cookie layer

• ¾ cup/170 grams unsalted butter, cut into small pieces and softened, plus more for greasing

• ¾ cup/165 grams brown sugar, light or dark

• ⅓ cup/66 grams granulated sugar

• 1 large egg, at room temperature

• 2 teaspoons vanilla extract

• ¾ teaspoon kosher salt (such as Diamond Crystal) or ¼ teaspoon table salt

• ½ teaspoon baking soda

• 2 cups/256 grams all-purpose flour

• 1 cup/170 grams chopped chocolate or chocolate chips (semisweet, bittersweet, milk or a mix)

• 34 chocolate sandwich cookies (such as Oreo), 10 of them halved into semi-circles 

For the brownie layer

• 1 ⅓ cups/227 grams chopped semisweet or bittersweet chocolate

• ½ cup/113 grams unsalted butter, at room temperature

• ⅔ cup/134 grams granulated sugar

• ½ cup/110 grams dark brown sugar

• 2 teaspoons vanilla extract

• 1 teaspoon kosher salt (such as Diamond Crystal) or ¼ teaspoon table salt

• 2 large eggs, at room temperature

• ½ cup/48 grams unsweetened cocoa powder, preferably Dutch-process

• ¼ teaspoon baking soda

• ½ cup/64 grams all-purpose flour

• Flaky sea salt, for finishing

1. Step 1

   Heat the oven to 350 degrees. Butter a metal 9-by-13-inch baking pan and line with parchment paper so there’s overhang on the long sides, then butter the parchment. Fill a medium saucepan with about 2 inches of water and bring to a simmer over medium heat (you’ll need it in Step 4).

2. 1. Step 2

      Make the cookie layer: As the water comes to a simmer, in a large heatproof bowl (metal preferred), mix together the butter, brown sugar and granulated sugar with a flexible spatula or hand mixer, until they’re just combined. Whisk in the egg, vanilla extract and salt until it’s slightly lighter in color, about 1 minute. Whisk in the baking soda, then switch back to the flexible spatula and fold in the flour until it’s mostly combined, with a few streaks of flour remaining. 

3. 1. Step 3

      Fold in the chopped chocolate and spread the cookie dough evenly across the prepared baking pan (reserve the mixing bowl). Top with the Oreos in an even layer, arranging the whole cookies in a four-by-six grid in the middle, then nestling the halved cookies around the edges so the cut sides touch the pan edges and only minimal gaps remain. Set the pan aside while you make the brownie batter.

4. 1. Step 4

      Make the brownie layer: Rinse out the mixing bowl (it doesn’t need to be scrubbed) and wipe dry, then add the chopped chocolate and butter. Set that bowl over the pot of simmering water and gently heat, stirring occasionally, until everything is melted and evenly combined, 2 to 4 minutes. Remove the bowl from the heat and turn off the stove. 

5. 1. Step 5

      Add the granulated sugar, dark brown sugar, vanilla extract and salt to the bowl, and whisk for 1 minute, until the mixture is smooth and the sugars have started to dissolve. Add the eggs one at a time, whisking thoroughly after each addition, and continue whisking for another minute, until the batter is roughly the consistency of marshmallow fluff. Add the cocoa powder and baking soda, and whisk for 1 minute, until they’re fully absorbed and the mixture is visibly thicker. Fold in the flour with a flexible spatula just until no streaks remain. 

6. 1. Step 6

      Scrape the brownie batter on top of the cookie batter and smooth into an even layer. Set a kitchen towel down on a work surface close to the oven.

7. 1. Step 7

      Bake for 20 minutes, until the top looks shiny and puffy. Gently rap the pan a couple times on the kitchen towel-lined surface, then top with flaky salt and put it back in the oven for 14 to 16 minutes to set the salt and finish baking. Take the pan back out of the oven and rap it once more to ensure the fudgiest brownie texture possible. 

8. 1. Step 8

      Cool completely at room temperature, about an hour, then slice into squares. For the cleanest slices, refrigerate the cooled brownies for about 30 minutes before cutting.

or to mark this recipe as cooked.