Soak the gelatine in cold water. Whip the cream until stiff. Puree the plums with juice and cinnamon.
Put half of the plum puree in a saucepan and heat. Dissolve the squeezed gelatine sheets in it. Mix the mascarpone and brown sugar together vigorously, add some of them to the plum puree and stir into the still warm puree. Refrigerate.
When the mixture begins to gel, fold in the whipped cream. Pour the mixture into glasses or dessert rings as desired and refrigerate for at least 2 hours.
For the sauce, season the remaining plum puree with lemon juice and sugar and serve with the mousse.