How to substitute beetroot for other ingredients into a cake recipe? Me and my roommate want to make a batch of brownies, however each of us has their heart set on a different recipe: I want to use one from Serious Eats because it's loaded with cocoa and I have a lot of faith in that site; he wants to use one that uses beetroot because he thinks it's healthier and someone gave him a few that we need to use up. (I thoroughly despise beets but I'm hoping that enough cocoa will cover them up here.)

Is there a way to get the best of both worlds? I've done cakes with shredded carrot and/or apples, and I assume the vegetables mostly contribute liquid and some structure. Is there some general rule I can use to replace some of the ingredients in a cake batter recipe for shredded beetroot other than trial and error?
There isn't a substitution formula, since beets add moisture, structure, and flavor to the batter.  Any substitution would therefore be complex and highly recipe-specific.

Instead, I suggest that you start with the BBC recipe and make use of the techniques in Stella Park's recipe, such as the brown butter, the foamed eggs, and the double chocolate, all of which could be added to the BBC recipe.

FWIW, beets are an excellent addition to baked chocolate desserts, making them moister, richer and darker.