In a medium mixing bowl, whisk the egg yolks until blended. Set aside.
In a medium saucepan combine the sugar, salt, cornstarch, flour and nutmeg. Whisk in the eggnog.
Cook the eggnog mixture over medium heat, stirring constantly until it thickens and starts to boil. (This takes about 10-15 minutes) Remove from the heat (leave the stove-top turned on).
While whisking constantly, slowly drizzle about half the hot mixture into the beaten egg yolks. Add the blended egg mixture back into the saucepan with the remaining hot custard. Return to the heat and cook 3-4 minutes more or until very thick and just starting to boil, stirring constantly to prevent scorching. Reduce the heat if needed.
Remove from the heat and blend in the butter, vanilla and rum. Pour the filling into the prepared crust. Cool to room temperature then refrigerate for at least 8 hours, or overnight.