I have like a devils food cake mix that I would like to turn into a dark chocolate cake. Can I just add Dark Cocoa to it...just a question?

Can anyone help?

My DIL is always talking about her Midnight cake and what a hit it is. Sounds super chocolaty from what she's said. Maybe if you google it you can find the recipe.
I haven't got her recipe, but I know she always doctors her cake mixes.

If you add up to 1/2 cup cocoa then add a pinch of baking soda too.

Thanks ladies