Lemon Filling: Crack eggs in a small bowl, beat slightly with a fork and set aside. 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.

Temper Eggs: 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 (warm them up so they don't scramble when added to the pot). Add egg mixture into the saucepan and stir well. Bring mixture to a gentle boil then cook for two more minutes. Remove from heat.

Finish Filling: 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.

Whipped Cream Topping: 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.