it takes abit of learning hun but this is how i do it and get min lumps (one or two) put water in your bowl as cold as you like have your hand blender plugged in and ready not tried it in anything else. put your shake on top of the water don't mix then add your mix a mousse on top of your shake and blend quickly going round for lumps.
another way i found works is to mix the shake and mousse in a bowl (the powders) then pour on the water and blend quick.
you have to be quick with blender and blend till you see no lumps.