In a large bowl, beat together the softened butter, brown sugar and granulated sugar until fluffy.
Then beat in the egg, additional egg yolk and vanilla extract. Note that you are not using 2 full eggs for the cookie dough layer. The extra egg yolk ensure a chewier cookie layer.
Then add in the flour, baking powder and salt. There’s very little baking powder in this recipe, because we want the cookie dough layer to be chewy and a little dense – more like the consistency of a brownie. Mix together starting with the mixer on a low speed to avoid the flour blowing everywhere.
Stir in the chocolate chips.
The cookie dough is then spooned into the prepared pan and smoothed into an even layer. Then add a layer of Oreos on top of the cookie dough and press them down lightly. I use 7 rows of 4 Oreos, plus extra cookies cut in half so that the entire pan is covered. You can, of course, also just use 7 rows of 4 cookies.
From there, you’ll make the brownie batter. This recipe is, again, from scratch and uses both melted dark chocolate (or semi-sweet) and cocoa powder for the perfect rich and fudgy chocolate flavor.
Add the butter and finely chopped chocolate to a heatproof bowl and melt together in the microwave using 45-second intervals on medium power. Stir between each interval and repeat the process until smooth. Note that it’s extremely important that you never melt chocolate on high power – which is often the default – or the chocolate can become grainy or burn.
Ensure that the batter isn’t hot, then whisk in the eggs, sugar and vanilla extract. It’s important the the melted butter and chocolate aren’t hot – otherwise they can actually cook the eggs.
Then you’ll whisk the the flour, cocoa and salt into the butter. If your cocoa and flour are lumpy, I recommend sifting the ingredients first to avoid overmixing the batter.
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