Dissolve lemon Jell-O in 1 cup boiling water, then stir in ½ cup cold water and set aside.
In a saucepan whisk egg yolks, sugar, and cornstarch. Cook over medium heat while stirring until slightly thickened.
Remove from heat and whisk in gelatin mixture, butter, and lemon juice until smooth.
Pour lemon filling into the pre-baked pie crust and spread evenly.
Beat egg whites and cream of tartar until soft peaks form. Gradually add sugar and beat until stiff glossy peaks appear. Mix in vanilla extract.
Spread meringue over the filling making sure it touches the crust edges to seal.
Bake at 350°F (175°C) for 8–10 minutes until the meringue becomes lightly golden.
Cool the pie and refrigerate at least 3 hours before slicing and serving.
