With a hand mixer, beat cake mix, water, eggs, and oil until smooth. Pour evenly into two greased, round cake pans and bake at 350 for 25 - 30 minutes or until a cake tester comes out clean. Cool on wire racks.
Meanwhile, stir together sugar, cornstarch, and salt in a saucepan. Whisk in water and lemon juice.
Heat over medium high heat and cook and stir until bubbling and thick. Reduce heat to low and cook and stir for 2 minutes.
Ladle a small amount of hot filling into bowl of beaten egg yolks (reserve egg whites) and whisk (this is to "temper" the yolks or heat them to the same temperature as the hot filling). Whisk the yolks into the saucepan.
Increase heat to medium and bring to a boil for two minutes, stirring constantly. Turn off heat and stir in butter. Set aside to cool.
For meringue, place reserved egg whites in the bowl of a stand mixer fitted with the whipping beater attachment. Add in cream of tartar and turn on mixer to medium speed. Stiff peaks will form after a while, then gradually add in sugar as it's beating.
To assemble cake, place one layer on an oven safe plate. Spread lemon curd filling and then the second layer of cake.
Spread meringue over top and sides of cake. Place cake in the oven preheated at 350 for 15 minutes or until the meringue has browned on the edges. Cool, slice and serve!