Pre-heat the oven to 150°C.
Whisk egg whites in a large bowl starting on low speed for 2 minutes until foamy.
Increase speed to medium for 1 minute, then to high until stiff peaks form.
Whisk in sugar gradually on fast speed until the mixture is stiff and glossy.
Spoon 8 mounds onto a lined baking sheet and hollow out the centres.
Reduce oven heat to 140°C and bake for 30 minutes.
Turn off the oven and leave meringues inside until completely cold to dry out.
Fill each meringue with chilled chestnut purée.
Whisk mascarpone, fromage frais, sugar, and vanilla together and spoon over the purée.
Dust the tops lightly with icing sugar before serving.