Preheat the oven to 180°C/350°F/Gas 4, Gas Mark 6 with the middle shelf ready. Grease the baking tin, then line with baking paper with the paper overlapping the sides a little.
Melt the butter in a pan over a medium heat. When the butter has melted, remove the pan from the heat and add the grated chocolate. Leave to 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.
Add the flour, cocoa powder, salt and a third of the oreos and stir until fully combined, then pour the mixture into the prepared tin. Scatter the remaining oreos over the top, pressing them in slightly. Bake in the oven for 25–30 minutes. The middle should be very so slightly gooey.
Leave the brownies to cool in the tin. The top will sink and crack a little. Pull the brownies out using the overlapping paper and cut the brownies into squares. Dust with icing sugar.
Substitute oreos for toasted walnuts, pecans or sprinkle the brownies with honeycomb.