Prepare cake: Preheat oven to 350F/177C. Grease and line bottom of two 7-inch round pans. Set aside.
In a mixing bowl, combine cake mix, water, eggs and oil. Beat by hand using a wire whisk until well-blended.
Pour mixture into prepared pans and bake for 30 minutes or until toothpick inserted at the center comes out clean. Cool before frosting.
Prepare lemon curd: In a glass bowl, over a double boiler, whisk together egg yolks, lemon juice, lemon zest, salt and egg yolks.
Whisk constantly until the mixture thickens and coats the back of the spoon. Remove from heat then whisk in butter.
Transfer in another bowl then cover directly with cling wrap. Refrigerate to set.
Prepare frosting: In medium bowl, beat egg whites with electric mixer on high speed just until stiff peaks form.
In a saucepan, combine sugar, corn syrup and water until well mixed. Cover and heat to rolling boil over medium heat. Uncover and boil 4 to 8 minutes, without stirring, to 242°F on candy thermometer.
Pour hot syrup very slowly in thin stream into egg whites, beating constantly on medium speed. Add vanilla. Beat on high speed about 10 minutes or until stiff peaks form.
To assemble cake: Place one layer of the cake in a serving dish, spread a small amount of marshmallow frosting and top with lemon curd.