Preheat the oven to 350°F (175°C)
Prepare or place a 9-inch pie crust in a pie dish
Bake the crust until lightly golden if needed
In a saucepan, whisk together sugar, cornstarch, salt, water, lemon juice, and lemon zest
Cook over medium heat, stirring constantly, until thickened
Remove from heat
Whisk egg yolks in a bowl
Slowly add a small amount of the hot lemon mixture to the egg yolks while whisking
Return the egg yolk mixture to the saucepan
Cook for 2 to 3 minutes, stirring constantly
Stir in butter
Pour the lemon filling into the baked pie crust
In a clean bowl, beat egg whites and cream of tartar until foamy
Gradually add sugar while beating until stiff, glossy peaks form
Spread the meringue over the hot lemon filling
Seal the meringue to the edges of the crust
Bake until the meringue is golden brown
Cool the pie at room temperature
Chill before serving