Filling:

• While the pie crust bakes, whisk together the sugar, water, corn starch, egg yolks, lemon juice and lemon zest in a medium saucepan.
• Cook over medium-low, whisking constantly, until thickened. Filling will be thick, slightly thicker than pudding when warm. This can take 10-15 minutes, but don’t rush it. Remove from heat and stir in butter.
• Pour hot filling into the pie crust and make the meringue.

Meringue:

• Preheat oven to 375 degrees F.
• In a small saucepan, whisk together the water and corn starch. Cook over medium heat until thickened and clear. Set aside but keep warm.
• In a clean bowl with clean beaters, beat egg whites, vanilla and salt until soft peaks form.
• Gradually add in the sugar, beating until stiff glossy peaks form.
• Gradually add in the warm corn starch mixture, beating constantly, until completely incorporated and meringue is light and fluffy and holds stiff peaks.

Pie Assembly: 

• Spread meringue onto hot pie filling (it’s important that the filling is still hot!) and spread right to the edge of the pie crust. The meringue must touch the edges all the way around.
• Bake at 375 F until the tops of the meringue are light golden brown. Don’t over-bake!
• Let pie rest on the counter on a wire rack for 1-2 hours, then refrigerate for 5-6 hours until chilled, or overnight.
• Slice and serve.