Dissolve lemon gelatin in 1 cup boiling water. Stir until fully dissolved, then add cold water.
Whisk egg yolks, sugar, and cornstarch in a saucepan. Cook over medium heat while stirring until slightly thickened.
Remove from heat and mix 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 continue beating until stiff glossy peaks form. Add vanilla extract.
Spread meringue over filling, making sure it touches crust edges.
Bake at 350°F for 8–10 minutes until the meringue turns lightly golden.
Cool the pie completely, then refrigerate for at least 3 hours before slicing.
