Musili gets that thick dessert like quality from several things, the richness of the dairy (e.g. whole milk vs. skim), they types of grains and dried fruits in the mix (e.g. more dried fruit will produce a sweeter result) and the length of time the musili and dairy blend together before eating.
Typically, I want flavorful, not too sweet and very heathy, so I use a musili that is mostly grain and seeds with only a little dried fruit and nuts. I blend that with a combination of non-fat yogurt and low-fat milk at least 10-12 hours before I want to enjoy.
For something as rich as dessert, try a combination of greek full-fat yogurt and whole milk with organic musili brimming with a variety of dried fruits and nuts.
Best of luck