In a large saucepan combine sugar and cornstarch. Whisk in milk and fresh lemon juice. Cook over medium heat, stirring constantly, until mixture is thick and bubbling.

Reduce heat to medium-low. Add a spoonful of the hot filling into the bowl with beaten egg yolks and stir well. Repeat this process with two or three more spoonfuls of the hot mixture added to the egg yolks, to temper the eggs.

Add egg mixture into the saucepan and stir well. Bring mixture to a gentle boil then cook for two more minutes. Remove from heat.

Add softened butter and lemon zest and stir until butter melts completely. Allow mixture to cool for a few minutes, then stir in sour cream.

Add filling to baked pie shell. Place a piece of plastic wrap or parchment paper over the pie and refrigerate for at least 3 hours before serving.

In a mixing bowl, beat heavy whipping cream with an electric mixer. While beating, gradually add sugar. Continue beating until stiff peaks form. Spread whipped cream over cooled pie filling.