Combine sugar, flour, and salt in a saucepan. Gradually stir in milk. Cook and stir on medium heat until thickened and bubbly. Reduce heat, cook, and stir for 2 more minutes. Remove from heat.

Beat yolks slightly. Gradually stir 1 cup of custard mix into yolks. Return yolk mixture to the saucepan with the rest of custard and bring to a gentle boil. Cook and stir for 2 more minutes. Remove from heat and stir in butter, lemon juice, and zest. Fold in sour cream.

Pour filling into baked pie shell. Chill until set.