Melt the chocolate& butter in a heavy saucepan on very low heat, remove pan from heat Stir in the Chestnut puree, a little at a time, mashing with the back of the spoon to break up large pieces (small bits of puree will remain in the mixture) Add vanilla& salt.
Beat until stiff and fold in a 1/2 cup cream.
In a fresh bowl beat egg whites with the cream of tartar.
Heat sugar and 3 tbsp water to boiling, stirring until it reaches 240F degrees.
Slowly beat the hot syrup into the egg whites.
continue beating until stiff peaks form.
Gently fold in the chocolate mixture, making sure no streaks of egg white remain Pour into a 2 quart clear or ceramic bowl.
Cover and refrigerat for 4 hours or overnight.
To serve Beat the remaining cream with the icing sugar.
Pipe the cream on the mousse and garnish with cherries.