1. MAKE THE PIE FILLING: In a bowl, mix the lemon juice with the passion fruit puree, egg yolks and beaten whole egg. In a saucepan, whisk the sugar with the cornstarch. Add the water, whisking until smooth. Cook over moderately high heat, whisking constantly, until thick, translucent and bubbling, about 5 minutes. Remove from the heat. Whisk a few tablespoons of the cornstarch mixture into the egg mixture, then gradually whisk the egg mixture back into the saucepan. Bring to a boil over moderately high heat, whisking constantly for about 1 minute, until the filling is thick; remove from the heat. Stir in the butter and lemon zest and scrape the filling into the Buttery Baked Pie Shell. Let cool completely.
2. MAKE THE TOPPING: Preheat the oven to 425°. In a large stainless-steel bowl, using an electric mixer, beat the egg whites with the sugar, water and salt until well blended. Set the bowl over a saucepan filled with 2 inches of boiling water and beat the egg whites until the sugar is dissolved and the mixture is warm. Remove from the heat and beat until the meringue is stiff, glossy and cool, about 5 minutes. Spread the meringue evenly over the filling, swirling it decoratively. Bake the pie on the top rack of the oven until the meringue is deep golden and set, about 7 minutes. Transfer the pie to a wire rack in a draft-free place and let cool completely. Serve at room temperature or chilled.
Make Ahead: The pie can be refrigerated for up to 1 day.