Cocoa powder is an unsweetened powder produced by grinding the seeds of the fruit of a tropical evergreen tree called the cacao, or cocoa tree.
These seeds, or beans, are first fermented and then dried and roasted. After roasting, the beans have their hulls removed and are then processed to extract their fat, which is known as cocoa butter. The leftover solids are further ground into what eventually is sold as cocoa powder.
Cocoa butter is in turn one of the key ingredients in bar chocolate, along with sugar, cocoa powder and some form of emulsifier, such as lecithin, to hold them all together.
Note that not all of the fat from the cocoa butter is removed from the cocoa powder. Indeed, the higher quality cocoa powders have more fat in them. You can generally determine the quality of a cocoa powder by comparing its fat content as shown on the nutritional label.
Another fun fact: Cocoa powder is the missing ingredient in so-called "white chocolate," which is produced by combining cocoa butter and sugar (plus an emulsifier) but no cocoa solids.
More crucially, we should establish that cocoa powder is not the same thing as cocoa mix, or instant cocoa, which when combined with hot water or milk instantly produces a mug full of hot cocoa. This product, which usually comes in packets, contains cocoa, sugar, dehydrated milk and other ingredients, but is not what you would use to bake brownies or chocolate cake.
Likewise, don't try to make a cup of cocoa by simply adding hot water to unsweetened cocoa powder.
Dutch Process Vs. "Natural" Process
The two basic types of cocoa powder are Dutch process and natural process. You'll find them sold and labeled each way, and in addition you'll also see cocoa powders labeled "Dutch and natural blend" or something along those lines. But what are these two types of cocoa, and what does the term Dutch process refer to?
To begin with, pure ground cocoa powder has a pH level between 5.3 and 5.8, which is to say that it is acidic. It's perfectly edible, but the acidity affects its flavor, the way it interacts with other ingredients, how soluble it is, and other factors.
Ordinary supermarket cocoa powder, what's often referred to as "natural" or "Broma" processed cocoa powder, is exactly what is described above: ground cocoa beans that have undergone no processing to alter their pH levels. It's a lighter, almost reddish-brown color.
Dutch process (sometimes called "Dutching") is a process where after separating the cocoa butter, the beans are washed in an alkaline solution, produces a cocoa powder that is darker brown and less acidic, with a pH of between 6.8 and 8.1, which is chemically neutral.
The Dutching process produces a cocoa powder that dissolves more easily, which makes it easier to work with in recipes, especially in ice cream and chocolate drinks. It also has a milder flavor.
Note that Dutching reduces the antioxidant properties of cocoa, if you happen to take an interest in such things.
Why Would I Use Dutch-Process Cocoa?
The short answer is: Because that's what the recipe calls for.
For baking, the type of cocoa does matter, because the acidity of the cocoa powder might well be the only thing activating the leavening agent in the recipe.
If the recipe calls for baking soda, for example, natural cocoa powder is probably fine, because the acidity in the cocoa will activate the baking soda. If the recipe calls for baking powder (or both baking powder and baking soda), then it probably also calls for Dutch-processed cocoa powder.
But again, this is just background. So long as you follow the recipe, you don't need to figure any of this out by yourself.
Thus, if you're at the "haven't bought it yet" stage, simply buy whatever kind of cocoa powder the recipe you're planning to make calls for.
If, however, you're at the "I have some specific type of cocoa powder in my pantry, what can I use it for?" stage, then your task is finding recipes that use the specific kind of cocoa powder you've already decided to use.
And, good news! You can probably find a recipe for brownies or chocolate cake on the label of the container of cocoa powder you have in your hand. If not, the label will almost certainly feature a web address for a site that will provide recipes. The advantage to using recipes provided by the manufacturer of your cocoa powder is that you'll know, without guessing, that the recipes will work for that exact cocoa powder.
When Do I Use Cocoa Powder in Candy Making?
Cocoa powder is frequently used in fudge, but can also be a coating on chocolate truffles. It's also used in other candy recipes, such as Cocoa Mints, Chocolate Marshmallows, and Tiramisu Truffle Squares. One of the advantages of cocoa powder is its shelf life; it stores for up to a year if kept sealed in a cool, dry place.
For candy making, the types of cocoa powder can usually be used interchangeably, and you should use whichever cocoa you think tastes best.
Cocoa Powder as a Baking Chocolate Substitute
If you have a recipe that calls for melted unsweetened chocolate, it's easy to use cocoa powder as a substitute. For every 1 ounce of unsweetened chocolate called for in the recipe, replace it with 3 level tablespoons of unsweetened chocolate plus 1 tablespoon of fat—melted butter, margarine, or oil.
Substituting melted unsweetened chocolate for cocoa powder is a much harder task, and isn't recommended, as the percentage of fat and cocoa solids are difficult to replicate in a simple formula.