Sour Cream Lemon Pie

1 c.  sugar
1/2 c. flour
1/2 tsp. salt
2 c. milk
3 egg yolks
1/4 c. butter
1/4 c. lemon juice
1 1/2 tsp. lemon zest
1/2 c. sour cream
1  9-inch baked pie crust

In saucepan combine sugar, flour, and salt.  Gradually stir in milk.  Cook and stir until bubbly.  Reduce heat and cook and stir two more minutes.  Remove from heat.  Slightly beat egg yolks.  Slowly pour 1 cup of hot mixture into egg yolk while stirring.  Return to saucepan and bring to gentle boil.  Cook and stir two more minutes.  Remove from heat.  Stir in butter, lemon juice and peel.  Gently fold in sour cream.  Pour into pie shell and cool.  Serve with whipped cream if desired.