MAKE THE TOPPING
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 <a href="/recipes/buttery-baked-pie-shell">Buttery Baked Pie Shell</a>. Let cool completely.