Preheat Oven & Prepare Pan: Preheat oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper.
Make the Shortbread Crust: In a bowl, cream together butter and sugar. Add flour and salt, mixing until crumbly. Press into the prepared pan and bake for 15-18 minutes until lightly golden. Let cool.
Prepare the Lemon Filling: In a saucepan, whisk together sugar and cornstarch. Add water and cook over medium heat until thickened. Slowly whisk in beaten egg yolks and cook for 1-2 minutes. Remove from heat and stir in lemon juice, zest, and butter. Pour over the cooled crust and set aside.
Make the Meringue: In a clean bowl, beat egg whites with cream of tartar until foamy. Gradually add sugar and vanilla, continuing to beat until stiff peaks form.
Assemble & Bake: Spread meringue over the lemon filling, making decorative peaks with a spatula. Bake for 12-15 minutes until meringue is golden.
Cool & Serve: Let cool completely before slicing into bars. Enjoy!