For crust, in a mixing bowl, combine the flour, nuts, brown sugar and grated chocolate. Add melted butter and mix well. Press mixture into a deep-dish, 9-inch pie plate, or 10-inch pie plate to form a crust.
Bake in a 350 degree F oven for 12 minutes. Cool on wire rack.
In a small bowl, stir the gelatin into the cold water. Set aside.
In a medium saucepan, mix the granulated sugar, and cornstarch. Stir in eggnog. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more. Stir in gelatin until dissolved.
Divide mixture into 2 equal parts. Stir the melted chocolate into one portion. Stir rum into the remaining portion. Cover the surface of both with plastic wrap and cool completely.
Fold whipped cream into rum portion. Spoon the rum-flavored cream portion into the cooled piecrust. Top with the chocolate mixture. Using a knife, gently swirl the chocolate into the white portion. Chill in the refrigerator at least 4 hours or until firm. Makes 8 servings.