Combine the brown sugar, flour, and salt in the top of a double boiler.
Blend in 1/2 cup milk; mix thoroughly.
Gradually add the scalded milk, stirring until blended.
Bring rapidly to boiling and cook 3 minutes.
Set over boiling water; cover and cook 5 to 7 minutes, stirring 3 or 4 times.
Stir about 3 tablespoons of hot mixture into slightly beaten egg yolks.
Immediately blend into mixture in double boiler.
Cook and stir over boiling water 3 to 5 minutes.
Remove from water and blend in the butter and vanilla extract.
Cool slightly, stirring occasionally.
Cover; cool to lukewarm in refrigerator.
Turn filling into pie shell or tart shells.
Chill, if desired.
Top pie or tarts with sweetened whipped cream and toasted almonds or pecans.