Also... sometimes citric acid can come in larger granules (like the size of sugar), rather than the really fine powder that makes it fizz better. I stick mine in a coffee grinder and give it a quick blast, to reduce the size of the granules. I've been known to improvise with a rolling pin and a plastic bag (like you might with cookies for making cheesecake base), when I couldn't find the lid for the coffee grinder once, and that seemed to turn out ok. They fizzed a little slower, but they did fizz, and it was kind of nice, as it lasted longer before totally disolving.
I would usually go with 2 parts bicarbonate of soda, 1 part citric acid and 1 part other stuff for the dry ingredients, if that helps you with the formulating. No reason why you can't put loads of other cool and funky stuff in... I'd just reduce the amounts/proportions.
You won't need as much cocoa powder to make it chocolatey. To be honest, I'd use less, but suppliment it with a Chocolate fragrance oil, unless you want the bath water to turn really really brown... Using a bit less, combined with the goats milk in there will make a warmer creamier looking chocolatey brown in the bath water (assuming you don't add any other colours).
Another thing I do, (which you're welcome to steal!), is that I also add a few drops of Neroli essential oil when I make chocolatey things. It's renowned as a 'feel good' oil, and even though you might not conciously pick up on the smell, perhaps it helps to give you a bit of a mood boost the way eating a bar of chocolate might!
I might adjust the recipe to something like...
2 oz. Cocoa Butter
•2 oz. Sweet Almond Oil
•1/4 cup Rice flour
•1/4 cup Goat’s Milk Powder
•1 tbsp Pure Coco Powder
•3/4 cup Epsom Salt
•1/4 Cornstarch
•2 cup Baking Soda
•1 cup Citric Acid
•5-10 Essential Oil
I would melt the cocoa butter with the sweet almond oil in the microwave, then when it comes out and has cooled a little bit, add your essential oils or fragrance to that mixture, before combining it with the dry ingredients. This helps the smell to disperse well through the mix. The reason I add the fragrance after microwaving, is to avoid them evaporating away with the heat before I've started. I've completly stopped using witch hazel spritzes (which some people recommend) when making bath bombs, fizzies and such things, and instead use just oils and butters in this kind of way. I find the mixtures combine and set much better, plus the sweet almond oil and butter will have you coming out of the bath with lovely feeling skin! Sounds like a really luxurious combination you have going on there.
If you fancy getting creative with the colouring of them (though it will all blend in in the bath of course), you combine your dry ingredients minus the goats milk and cocoa powder. Then split the mix into two, so you have it in two different bowls, and add the cocoa powder to one, and the goats milk to the other. After you add your liquid ingredients (shared equally between the two mixtures), you can alternate or layer colours as you pack the mixture into your moulds. It's a lot easier than it probably sounds from that.
These bath bombs were done exactly that way (but I went kind of nuts and used 4 colours...!) and turned out looking really pretty. They have sweet almond oil and shea butter in as the main liquid ingredients. Once left overnight to set (on the table in front of my east facing kitchen window) they were pretty solid to handle, but fizzed beautifully when I used one in the bath myself.
I hope that helps!