Line baking sheet with parchment paper. Spoon egg white mixture equally into eight mounds on paper. Using the back of a spoon, make an indentation in the center of each mound.
Bake at 300° for 30 minutes. Turn oven off and leave door closed and let the shells dry in oven for an hour. Use a small spatula to carefully remove the shells from paper.
In a small saucepan, combine the cornstarch and remaining sugar and salt. Stir in water until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat and cook and stir 2 more minutes. Remove from the heat.
In a small bowl, beat egg yolks. Whisk a small amount of hot mixture into the egg yolks and then return the egg mixture to the pan stirring constantly. Bring to a gentle boil and cook and stir another 2 minutes. Remove from the heat and gently stir in the lemon juice, zest and butter. Cool to room temperature without stirring. Place a piece of plastic wrap right on top of the custard and chill.
Just before serving, fill meringue shells with custard and top with sweetened whipped cream.
You can make these smaller if you'd like. I've done that before and made 12 meringue cups instead of 8. My boys love anything involving a meringue shell, so they love the bigger size, but making 12 instead of 8 still makes a perfect size dessert.
The shells can be made ahead of time and stored in an airtight container, or sometimes just leave them on the cooling rack and cover them with a towel. I like to make them the night before so that's one less thing I need to worry about.
I make the custard filling either the night before or the morning of the day I'm serving them. It takes several hours to chill so making it the night before gives it plenty of time to chill. Then I just whip some cream just before I assemble them. I usually will assemble them n hour or two before I'm ready to serve them and store them in the refrigerator until I need them.