Every time I try to mix a large amount of cocoa powder (ie. enough for a cup of hot chocolate) into a large amount of hot water (ie. a cup full), I end up with a significant amount of "cocoa sludge" where the powder doesn't dissolve properly and turns into a big, clumpy mess.
It doesn't matter if I pour the cocoa in first and pour hot water on top of it, or if I pour cocoa powder into a cup full of hot water. It doesn't matter if I stir or not (stirring can even make it worse; cocoa sludge sticks surprisingly well to plastic spoons!) or even if I pour the cocoa powder in very slowly, a little bit at a time, to give it time to dissolve. I still end up with a bunch of cocoa sludge no matter what I do. Often, stirring can even make a protective layer of cocoa sludge form in a bubble around pristine cocoa powder, which then needs to be broken open so it can dissolve.
Does anyone know any good tricks to making all of the cocoa powder dissolve properly and emerging out the other side with a nice, smooth cup of hot chocolate?