Heat the half-and-half, pumpkin puree, brown sugar, salt, cinnamon and nutmeg in a heat-proof bowl set over a pan of simmering water until hot, about 5 minutes.
Whisk the egg yolks in another bowl, stir some of the hot pumpkin into the egg yolks to heat them. Pour the egg-pumpkin mixture slowly back into the double boiler and stir well.
Heat the mixture again over the simmering water for another 4-5 minutes, until it begins to thicken (about 140 degrees), stirring CONSTANTLY. You don't want the eggs to scramble. Remove from heat.
Dissolve the gelatin in 1/2 cup cold water. Add the dissolved gelatin, bananas and orange zest to the pumpkin mixture and mix well. Set aside to cool.
Whip the heavy cream (chilled very cold) in the bowl of an electric mixer with a whisk attachment until soft peaks form. Add the sugar and continue to whisk until you have firm peaks. Carefully fold the whipped cream into the cooled pumpkin mixture and pout it into a 7 inch x 2-3/4 inch round souffle dish.
Chill for 2 hours or overnight.
For the decoration: Whip last 1 cup of cold heavy cream until soft peaks form. Add the sugar and vanilla and continue to whisk until you have firm peaks. Pipe or spoon the whipped cream decoratively on the mousse and sprinkle, if desired, with remaining orange zest.
[As always with zest, be sure to just get the orange skin and not any of the white pith, which is bitter -- the orange in this enhances the flavors so that it is very refreshing - I could NOT believe how good this was - and how satisfying a small portion was!].