4: Take 1/4 of the egg whites and pour over the mascarpone + chestnut cream mixture.
5: Mix gently with a whisk, but don't be too insistent. Just aerate the mixture slightly to make it easier to incorporate the beaten egg whites.
6: Take half the remaining egg whites and pour over the previously aerated mascarpone + chestnut cream mixture. Fold in very gently, this time with a spatula, following these instructions.
7: Do the same with the other half of the egg whites, until you obtain a fine mousse.
8: Divide between glasses or ramekins. If possible, use a piping bag with a fluted tip for a more attractive effect.
9: Refrigerate for at least 2 hours before serving.
Remarks
If you prefer a lighter mousse, use 4 egg whites instead of 3. For a little pep, you can add a dash of whisky, or other alcohol, to the mascarpone + chestnut cream mixture.