If one has a glass of hot water (that was heated before Shabbat) are they allowed to put hot cocoa powder in it on Shabbat? In this case, it’s a solid being absorbed by the liquid, but would it still present an issue of bishul since the cocoa powder is likely transformed in it?
Although the original psak deals with soup-powder, I think it would be no different with cocoa-powder.
Refer to the answer here on Dinonline.org:
it would be permitted to put instant soup mix into a kli shelishi. Even if some of the ingredients (i.e. spices, flavorings) in the soup mix were not cooked before Shabbos. Although they may not be put into a kli sheini, they are still not considered “kalei habishul” items that cook easily and are permitted in a kli shelishi. As a side point, it is important to note that there are some instant coffees, nowadays that contain 15% raw coffee beans, to give the coffee a freshly brewed taste. Such coffee may not be made in a kli sheini, but only in a kli shelishi.