What is the difference between the 2? Which is the best for Chocolate Buttercream
Dutch processed cocoa had been alkalized (lower acidity). In baking, this makes a difference, from a chemistry standpoint. It's also more soluble in liquids than natural cocoa (which makes it ideal for chocolate flavored drinks and sauces).
As far as using it strictly as a flavoring agent(as in buttercream)--without taking the solubility factor into account--it's reallu just a matter of preference. Dutched cocoa tends to have a milder (and some say smoother) flavor than natural cocoa. I say try both and see which you prefer.
I personally prefer make chocolate buttercream with melted chocolate as opposed to cocoa powder.
Dutch processed cocoa is soooooooo good in cakes! I threw out my can of Hershey's cocoa once I discovered it. I use it no matter what kind of cocoa the recipe calls for, although I might need to study up on other appropriate recipe alterations since the acidity is different, as PinkZiab mentioned.
ITA with the idea to use melted chocolate rather than cocoa powder for your bc, but if you have to use cocoa powder, use the good stuff . It's really cheap at amazon.com right now, because they are having a special til Aug 31: only 99c to ship a "grocery" order. Works out to about $4 per 8oz which is what you pay for Hershey's Dark anyway.