Cocoa powder results of the crushed cocoa beans, totally or parcially lacking in cocoa butter. It is produced by reducing the cocoa butter up to the powder texture point.
Cocoa powder usually has its fat contents under 20% of cocoa butter.
The aim of the cocoa powder is to be dissolved in milk. It is produced with a cocoa proportion range from 25% to 32%, and it is presented more or less reduced-fat.  
One of its utilities is as decoration in confectionery.
Cocoa powder is not easily soluble in water or milk.
The lesser it has fat content, the lesser its solubility will be.
Cocoa powder is usually used as an intermediate step in chocolate jams production, products for spreading, creams, syrups... The most popular version includes drinks making, since it is normally mixed with milk (milkshakes), so that the milk tastes of chocolate. It is also used as decoration in confectionery, such us Tiramisu, or drinks decoration like mocha coffee, capuccino, etc. .
There is a Spanish version called cacao a la taza, which means drinking hot chocolate to be served in a mug. Sugar and starches or flours are added to the cocoa powder in order to obtain a hot and thick drink, the traditional Spanish drinking chocolate.