Melt chocolate - Add the butter and chocolate to a heatproof bowl and microwave for 30 seconds at a time, stirring each time until melted. Alternatively you can do this using a double boiler. Set aside to cool slightly.
Combine dry ingredients for brownie layer - To a small mixing bowl (same one as the cookie dough ingredients) add the flour, cocoa powder and salt and use a whisk to combine.
Cream eggs and sugar - To the large mixing bowl you used earlier, add the eggs, both sugars and vanilla extract. Use an electric hand mixer to whisk until pale, fluffy and doubled in volume, about 2-3 minutes.
Add coffee - Add the coffee shot and mix until combined.
Add chocolate mixture - Next, add the cooled chocolate mixture and whisk until well combined. Use a spatula to scrape the bowl down.
Add dry ingredients - Add the dry ingredients and fold using the spatula until no dry ingredients are showing.
Layer the dessert - Spread the Oreo cookies across the top of the baking tin with the cookie dough. Pour the mixture on top and spread evenly.
Bake - Bake for 22-25 minutes or until a toothpick inserted into the center comes out with moist crumbs.
For super clean slices - Take out of the oven and allow to cook at room temperature. Then transfer to the fridge to cool for 2 hours before using a large knife to slice up. The cooling time is really important because it allows the slice to firm up. Run a sharp knife under hot water and dry off before slicing. Do this with each slice!