Special equipment: a pastry bag
For the meringue: Preheat the oven to 200 degrees F. Fill a medium saucepan with about 2 inches of water and bring to a gentle simmer over medium heat.
Prepare the bowl of a stand mixer by thoroughly wiping the inside with a paper towel dampened with white vinegar (see Cook's Note). Add the sugar, cream of tartar and egg whites. Place the bowl over the simmering water, making sure the bottom of the bowl does not touch the water. Whisk constantly until the sugar has completely dissolved, 5 to 7 minutes. To check if the mixture is ready, rub a bit of it between two fingers; if you do not feel any grains of sugar, the sugar has dissolved. Make sure to whisk constantly during this step; if left unattended, the bowl can become too hot and the eggs will start to cook.
Transfer the bowl to a stand mixer fitted with the whisk attachment. Whip for about 10 minutes, starting on low speed and gradually increasing to high, until the egg whites increase in volume and form glossy, stiff peaks.
While the meringue is whipping, line three baking sheets with parchment paper. On one sheet, draw a 10-inch circle, on another draw an 8-inch circle, and on the last draw one 6-inch circle and one 4-inch circle.
Using a pastry bag or offset spatula, spread the meringue over each traced circle to form a disc. For the smallest circle of meringue, which will be the top of the cake, use a large spoon to dollop the remaining meringue onto the parchment so it has some height. Bake until the meringues are dry and can be easily lifted from the parchment paper but are still white in color, 2 to 2 1/2 hours--the larger discs of meringue will take longer to bake. Let cool on the sheets placed on wire racks for at least 2 hours.
For the filling and garnish: When ready to assemble the dessert, whisk together the heavy cream and vanilla bean seeds in a cold bowl until soft peaks form.
Place the largest meringue disc on a serving platter. Top with a third of the whipped cream and sprinkle with a third of the passion fruit and pomegranate seeds. Top with the second largest meringue disc and add another layer of whipped cream and seeds. Repeat with the third layer and top the cake with the smallest disc. Decorate the edges of the cake with the mint, red currants and husk cherries if using. Serve immediately.
For this recipe it is critical that you use a very clean bowl, free of grease, to whip your egg whites--this is why you clean the bowl with vinegar. Do not skip this step or all your time and egg whites will go to waste!