First, make the pavlovas. Preheat the oven to 100 degrees Celsius and line a baking sheet with parchment paper.
Mix together the cream of tartar with a couple tablespoons of the 300g of caster sugar. Whip the egg whites until foamy, and soft peaks are beginning to form.
Sprinkle over the sugar, a few tablespoons at a time, beginning with the cream of tartar sugar. Continue to whip the egg whites and sugar until stiff, glossy peaks form. Be careful not to overwhip.
Gently fold in the coconut and divide into four mounds on the parchment paper. Using a spoon, create little depressions in each meringue for the cream.
Bake in the peheated oven for 1 hour, 25 minutes. Turn off the oven, and allow to cool to room temperature.
To make the passionfruit sauce, stir together the 150g sugar and coconut water in a small saucepan. Bring to a boil, and stir until the sugar is dissolved. Remove from the heat, and stir in the passionfruit seeds.
Shortly before you're ready to serve, whip the cream to medium peaks and mound on the meringue shells. Add the mango cubes on top, the coconut flakes (if using), and the passionfruit sauce.