Bake and cool a single 8-9 inch pie crust.
For the filling:
Combine the sugar, cornstarch, flour and salt in a large, heavy-bottomed saucepan. Gradually stir in water until smooth. Cook, stirring constantly over medium-high heat until thickened. Reduce heat and cook, stirring constantly, for an additional 2 minutes.
Remove pan from the heat. Ladle out a small amount of hot filling and slowly stir it into the egg yolks. Repeat two more times, then pour the egg mixture into the pan, stirring constantly. Bring to a gentle boil. Cook and stir 2 more minutes. Remove the pan from the heat. Stir in cubed butter and lemon zest. Gradually stir in lemon juice until combined. Pour into the baked crust.
Allow to cool on a wire rack then refrigerate until firm enough to slice. Garnish with fresh whipped cream.