Combine sugar, cornstarch and salt in top of a double boiler.
Slowly add milk,
stirring constantly. Place over gently boiling water and cook 10-15 minutes, stirring
constantly until mixture thickens to a loose custard.
Blend 1/2 cup of the hot sugar mixture into the egg yolks, 1 tablespoon at a
time.
Slowly pour and beat the warm egg mixture back into the remaining sugar mixture.
Continue cooking 3 to 5 minutes, until the custard is thick.
Add butter; cool to
room temperature.
Stir in the Kahlua and all but 1 tablespoon of the nuts.
Fold in 1 cup whipped
cream.
Fill pie shell, garnish with remaining whipped cream and reserved nuts.