Base: In a medium bowl combine walnuts, biscuit crumbs, sugar and melted butter. Mix thoroughly then press firmly into a large pie dish. Bake in preheated oven for 8 minutes. Remove and cool.
Chocolate Layer: Place water in a small bowl and sprinkle gelatine over top. Set aside to soften. In a medium saucepan mix together sugar, cornflour and salt. Whisk in milk and cook over low heat, stirring constantly. When mixture comes to a boil remove from heat.
Place whisked egg yolks in a small bowl and gradually add 1/2 cup of hot milk mixture while whisking constantly. Whisk egg yolk mixture back into hot milk mixture. Return saucepan to heat and allow to boil 3 minutes stirring constantly. Remove from heat and place 1 cup of mixture in a small bowl.
Place chocolate pieces in a glass bowl and microwave at 30 second intervals stirring at each interval until chocolate is melted and smooth. Stir chocolate and coffee into separated custard. Allow this mixture to cool then spread into bottom of crust. Place in refrigerator.
Rum Layer: Stir softened gelatine into remaining custard. Place a piece of plastic wrap directly on surface of custard and chill until firm.
In a large glass or metal mixing bowl whisk egg whites with cream of tartar until foamy. Gradually add sugar continuing to beat until whites form stiff peaks. Whisk rum into chilled gelatine mixture then fold in whipped egg whites. Spread mixture on top of chocolate layer. Chill pie at least 2 hours until filling is set.
To Serve; Immediately before serving whip cream until soft peaks form. Place whipped cream in a piping bag with a star tip and make 8 decorative swirls around edges of pie. Place a walnut half in the middle of each swirl. To cut pie, dip sharp knife in hot water.