In a medium saucepan, whisk to combine the cornstarch, water, 1 ⅓ cups sugar and salt. Bring to a boil over medium heat, stirring frequently, and boil for 1 minute. Remove from the heat and add about ¼ of the hot mixture to the egg yolks. Whisk well. Add the rest of the mixture to the eggs gradually, whisking well. When all the hot mixture is added to the egg yolks, pour the mixture back into the saucepan, turn the heat to low and cook for 1 more minute, stirring constantly. The mixture should be thick. Take mixture off the heat and gently stir in the cold butter, lemon juice and lemon zest.
Put mixture into a bowl and cover with plastic wrap, placing the plastic wrap right on the surface of the lemon mixture. Refrigerate until cold – about 2 hours.
Whip heavy cream with one teaspoon sugar until stiff peaks form but do not over mix.
Just prior to serving, pour the lemon mixture into the meringue pie crust. Top with whipped cream. Garnish with lemon slices if desired.