I've found a source of powdered carob. I've read that it is a substitute for cocoa, and I was wondering how this works out practically. Is it a one-to-one substitution, or should I only use it in conjunction with cocoa?
While you can certainly substitute powdered carob for cocoa, the flavors are distinct enough that you will likely need to adjust your ratios and sugars to make carob blend into a recipe better. Use less sugar than you would for powdered chocolate.
I found this description http://www.natural-health-restored.com/what-is-carob.html of Carob, and this comparison http://www.natural-health-restored.com/carob-vs-chocolate.html of carob vs. chocolate.
A note about the source http://www.natural-health-restored.com, I have never used this site personally, and can't vouch for its accuracy. It also seems to be centered around Vegan eating, so read into the site's commentary what you will.
Additionally, with powdered chocolate, you can never get quite the same reconstituted flavor with other fats/oils as you can with dark solid chocolate (the fats/oils are different from the solid chocolate's original fats). I suspect the same to be true for Carob, however I have no proof.
So (without exact ratios):
Personally, I used carob at a 1:1 ratio and found my carob muffins to be drier (more oil?), stronger in flavor (less carob powder?), and a touch on the sweet side (less sugar?).
My suggestion would be to find a recipe made for carob instead of just substituting in one for the other.
You can substitute carob powder one-for-one in recipes that call for cocoa powder. I do it all the time for my husband and son, both of whom are allergic to chocolate.
That being said, carob doesn't melt the way chocolate does, so trying to make ganache just doesn't work (been there... makes a MESS!)
I have made ganache with carob chips with great success after a few trial and error and the flavor is wonderful. What you need is to heat your heavy cream first and then gradually add the carob chips slowly until you get the thickness you want. Hint: you will not need as much carob chips as you would chocolate. You use less.
Hope this helps! ;)