Why do I have to chill a mousse for 8 hrs.
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Mousse that's designed to be a component for a cake has to set in order to be sliceable....all refrigerators are different so I suspect the writer wants to ensure that everybody making the cake is going to chill the mousse completely in order for the recipe to be successful. If the mousse doesn't fully set, it will run when you try to unmold/slice the cake.