To make the filling: Mix the sugar, Instant ClearJel (or cornstarch), and salt together in a saucepan., Add the eggs and whisk well, then whisk in the juice., Heat the mixture over medium heat until it's very hot; it may come to a boil, or it may just "seethe" a bit, like it's trying to boil and not quite making it. Let it boil or seethe for 30 seconds., Stir in the butter, stirring until it melts, then pour the mixture into the baked pie shell., Rub a piece of butter over the surface to prevent a skin from forming. Cover with plastic wrap, and chill for several hours before topping with the meringue., To make the meringue: Preheat your oven to 375°F with the oven rack in the lowest position., Whisk the meringue powder and water together, then beat until foamy., Add the salt, sugar and vanilla, and beat on medium-high speed until the meringue is thick and shiny., Beat in the marshmallow., To finish the pie: Remove the pie from the refrigerator., Top it with the meringue, using a spatula to form peaks., Bake the pie for 18 to 22 minutes, or until the meringue has browned., Remove it from the oven, cool it, and refrigerate until serving., Store the pie, loosely covered, in the fridge for up to 3 days.