Adjusting 'plain' cake recipe to make it chocolate... Total novice needs advice please!

[ojbsmum]

I am rubbish at baking but have found one recipe which has worked for me every time - the buttermilk birthday cake from How to be a Domestic Goddess (if only!)
I know that if I want to make a plain cake chocolatey I use cocoa powder but I assume it's not in addition to the flour? How much do I substitute? Does it really matter? I want the cake to be yummy and chocolatey but not taste of cocoa...
It's for my daughter's birthday so want it to be lovely! (Two weeks ago I attempted to bake a cake for my son's birthday. Three failed attempts later I bought one...)
Please help!

Weigh out the flour, take 2 tablespoons out, and replace with 2 tablespoons of cocoa.

That simple?! Great, thanks... Will crack on with it.
Then just the decorating to do - grumpy lion and cheeky monkey, not too much to ask!

I was going to say the same as CMOT! Yes, it is that simple

Yes that's how my DMum does it and she makes the worlds yummiest chocolate cake.

Must be coco, drinking chocolate has sugar and things and doesn't work anything like as well.