What can I substitute for butter in a custard-based pie? This weekend I made a great buttermilk pie.  Creamed butter/sugar add eggs / flour flavoring buttermilk and bake.

I want to reduce the calories.  Is there a substitute that I can use with this custard like pie?  

The recipe uses 1 stick of butter to 1 3/4 cups sugar

This is the complete recipe
Buttermilk Pie


1 3/4 C. sugar 
1 stick unsalted butter (1/2 C.) room temp.
3 large eggs
1/4 tsp. vanilla extract
pinch of salt
3 T. all purpose flour
1 C. buttermilk
1 9" pie shell, unbaked


Cream together the sugar and butter till well blended. Beat in the eggs, 1 at a time, until the mixture is nice and smooth. Add in the vanilla, salt and flour. Beat in the buttermilk until well combined. Fill the pie shell with the buttermilk mixture, just to the top of the shell. Bake at 300 degrees for 1 hour 10 minutes – Let rest before serving
Butter in this type pie is there to add "richness". It is possible to leave it out entirely and save about 800 calories, but it may not satisfy your definition of a good dessert. If you reduced the amount to 1/2 a stick (1/4 cup), you would reduce the total calories by about 400 and still have a very nice pie. 

I would not recommend replacing the butter with a reduced calorie margarine or spread, as they contain water and other stuff that will not improve the pie.