In the most basic sense a mousse is a base folded into whipped cream. This base could be made of just chocolate for the most basic mousse, or could be a custard like vanilla crème anglaise with gelatin added. A traditional mousse contains egg yolks whipped with sugar for extra volume, but modern versions often omit this step. The base also needs to include something that will “set” the mousse when chilled like chocolate or gelatin.