Whisk sugar, flour, corn starch & salt in a medium saucepan on medium-high heat. Stir in water, juice & zest until it comes to a boil. Stir in butter. In a separate dish, whisk egg yolks. Mix a 1/2 c of the egg mixture into the yolks. Pour yolk mixture into the hot sugar/lemon mixture {this ensures that the eggs won’t scramble}. Bring to a boil & continue stirring until thick. Remove from heat & pour into jars.
Preheat oven to 325 degrees. Mix corn starch & water in small saucepan. Bring to a simmer, whisking until thickened. Remove from heat when translucent & set aside. In a separate mixing bowl, mix cream of tartar & sugar. In a large mixing bowl, beat egg whites until frothy & add vanilla. Gradually add dry sugar mixture. Then add cornstarch mixture in a tablespoon at a time until stiff peaks form. Add to the top of your lemon mixture {optional: pipe it on with a frosting tip & bag} & bake until golden brown, about 15 minutes.
Notes
Be sure to let your curd get really thick, you want it to be a smooth paste. Don't be tempted to take it off of the heat too early.
Use the freshest eggs you can. They are more stable than older eggs and trap air more easily.
Chill your eggs before you separate then. It makes the yolks stronger and they are less likely to break.
Don't over beat your meringue, it should be stiff and glossy. If it breaks down or separates it is over beaten.