Preheat your oven to 180C/160C Fan, and line a 9" square tin with baking parchment.
In a heatproof bowl, add your 200g of dark chocolate, and the 200g dairy free spread. Melt in the microwave, or on a double boiler. Mine took 90 seconds to melt, in 30 second bursts. Cool for a few minutes.
In a large bowl, or a stand mixer bowl, add the eggs and the caster sugar. Whisk together for about 3-5 minutes. You want it to thicken, get paler, and become mousse like. It will make a trail that lasts a couple of seconds when you lie the whisk out of the mix.
Once the egg/sugar mix has been whisked, fold in the chocolate mixture. Be careful not to knock the air out.
Add in the plain flour and cocoa powder, and fold again carefully.
Finally, add in the chocolate chunks and the chopped Oreos, and fold a few times. Pour into the tin, and bake in the oven for 25-30+ minutes.
Once baked, leave to cool in the tin, or on a wire rack.
Notes
I used Vitalite spread, as its the one I had in the fridge. You can use any other, or even the dairy free Stork (the one wrapped in foil).
The cocoa powder I used was a supermarket own on, that is naturally dairy free. Same with the dark chocolate.
I used regular oreos, but use any flavour you like thats also dairy free!
Always double check the ingredients you are using, but I only had to change the spread/butter part of the brownies.
Please be aware that Oreos and dark chocolate "MAY contain milk" or other dairy ingredients due to cross contamination at factories for example. The ingredients list itself however for the products I used, such as the Oreos, are dairy free. Please check whatever ingredients you pick up thoroughly if you have a severe allergy.
Some brownies can take longer to bake in the oven, keep going if they aren't quite done. If after an hour they haven't finished, take them out, cool, and store in the fridge till cutting and it'll be fine.