Preheat the oven to 350 degrees F (180 degrees C) with the middle shelf ready. Grease the baking pan, then line with parchment paper with the paper overlapping the sides a little.
Melt the butter in a pan over medium heat. When the butter has melted, remove the pan from the heat and add the grated chocolate. Let stand for a few minutes until the chocolate goes soft, then stir together. Alternatively, you can put the chocolate and butter in bowl and melt in the microwave in 25-second blasts, stirring well each time.
Whisk the eggs, egg yolks, and vanilla together in a large bowl until they begin to get light and fluffy. Add the sugar in two additions, whisking between each. Pour it around the side of the egg mix so as not to knock out the air that has been whisked in to it. Keep whisking until the mixture becomes stiffer. Once the egg mixture is ready, pour the chocolate into it, again around the sides so as not to knock the air out.
Sift the cocoa into a large bowl and add the flour, salt, and a third of the Oreos. Stir until fully combined, then pour the mixture into the prepared pan. Scatter the remaining Oreos over the top, pressing them in slightly. Bake in the oven for 25 to 30 minutes. The middle should be very so slightly gooey.
Let the brownies cool in the pan. The top will sink and crack a little. Pull the brownies out using the overlapping paper and cut the brownies into squares. Dust with confectioners' sugar.
Cook's Note: Substitute Oreos for toasted walnuts or pecans or sprinkle the brownies with honeycomb.
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