Beat the yolks together with the brown sugar in a glass or metal bowl. Add the maple syrup, set over a water bath, and cook, whisking, until the mixture reaches custard consistency and coats a spoon, about 15 minutes. It should be quite thick. Remove from the heat and stir in the gelatin. Set aside to cool completely. This is important: if the mixture is warm it will deflate the egg whites.
In a separate bowl, beat the egg whites to peaks. In another bowl, do the same with the heavy cream. Fold them alternately into the custard mixture. Turn the mixture into a serving bowl. Cover, and chill for at least 4 hours before serving. Or, alternatively, you could pipe the mousse into parfait glasses and chill.