Step 1

Combine sugar and cornstarch in a saucepan. Stir in milk until smooth. Cook and stir over medium heat until thickened and bubbly, about 10 minutes. Reduce heat; cook and stir for 2 minutes more. Remove from heat.

Step 2

Stir a small amount of hot filling into the egg yolks; return all to the pan. Bring to a gentle boil; cook and stir for 2 minutes. Remove from heat; stir in butter and lemon zest. Gently stir in lemon juice. Cool to room temperature, about 30 minutes.

Step 3

Beat cream in a chilled glass or metal bowl with an electric mixer until soft peaks form.

Step 4

Fold whipped cream and sour cream into the lemon filling. Pour into the prepared pastry shell. Chill for at least 2 hours before serving.