For the crumb crust: combine the nuts, flour, salt, sugar, and cinnamon in a mixing bowl, and stir to mix well. Stir in the melted butter and continue stirring until the mixture has absorbed the butter. Break the mixture into even 1/2 to 1/4 inch crumbs, rubbing with the fingertips. Place half the crumb mixture into a 9 inch Pyrex pie pan and press with the fingertips to line the pan evenly. Place the remaining crumb mixture, in an even 1/2 inch layer on a cookie sheet. Bake the crust and crumbs on the middle rack of the oven about 20 minutes, until crisp and light golden in color. Cool the crust and crumbs on racks.
For the Mousse Filling: Whip the cream until it holds soft peaks and set it aside in the refrigerator. Sprinkle the gelatin on the water in a small, heatproof bowl. Allow to soak 5 minutes, then place over a small pan of simmering water to melt while preparing the filling.
When the gelatin is melted, remove from the pan and allow to cool.
In the bowl of an electric mixer, or another heatproof bowl, whisk the egg yolks. Whisk in the rum, then the sugar. Place over a pan of gently simmering water and whisk constantly until thickened, about 3 minutes. If the yolk mixture becomes too hot, it may scramble.
Remove the bowl from the water and beat by machine, on medium speed, until cooled to room temperature. Whisk in the dissolved gelatin, then fold in the whipped cream and the chopped nuts.
Pour the filling into the cooled shell and smooth the top. Cover loosely with plastic wrap and chill until set at least 6 hours.
To finish the pie, top with the baked crumbs. Or whip the optional cream, spread half on the mousse and top with the crumbs. Then, pipe a border of rosettes of the remaining cream around the edge of the pie with a pastry bag fitted with a star tube.