In a medium pot, whisk together the sugar, water, cornstarch and salt.
Cook over medium heat, stirring frequently until mixture boils.
Boil for one minute, stirring constantly, then remove from heat.
In a separate bowl, begin whisking the egg yolks.
While whisking, add 8-10 tablespoons of the cooked sugar mixture to the egg yolks, one tablespoon at a time.
Once 8-10 spoonfuls of the sugar mixture have been added, pour the egg yolk mixture into the saucepan with the remaining sugar mixture, whisking constantly.
Cook over medium-low heat for one minute, stirring constantly.
Remove from heat and whisk in the butter and lemon juice.
Cool to room temperature and stir in ½ cup of whipped cream.
Pour entire mixture into pre-baked pie crust.
Top with a thick layer of whipped topping and refrigerate for at least three hours before serving.