about cocoa...

Cocoa powder is just the cocoa solids minus the fat/oil & liquor.

Chocolate has the Cocoa liquor and the Cocoa fat/oil mixed back together, along with the other ingredients.

milk chocolate has the above plus milk fat & solids. The amount of Cocoa liquor added is less than for bittersweet or dark chocolate.

"Dutch-Processed Cocoa Powder" is Alkalized Cocoa Powder. This reduces its bitterness, but changes its effects when used with Baking Powder. Cocoa Powder and Dutch Cocoa Powder cannot be substituted for each other, without proper substitution...
Its the Dutch Cocoa Powder that is used for baking in European pastry.

After the specially aged cocoa fruit has matured, it is pressed. The liquid is called "liquor", the pulp-cake is known as the "solids".

http://www.joyofbaking.com/cocoa.html