Preheat the oven to 225º F. Clean the bowl and whip of an electric mixer, or a large glass bowl, and the beaters of a hand mixer with white vinegar and wipe dry.
Line a baking sheet with parchment paper. Using a 9-inch cake pan or plate as guide, trace a circle onto the parchment. Turn the parchment over so the ink side is facing down.
Place the egg whites in the prepared bowl and whip on low speed until foamy, 1 to 2 minutes. In a small bowl, whisk the sugar, cornstarch, and cream of tartar to combine.
Raise the speed to medium and begin to add the sugar mixture gradually, whipping to stiff peaks. With such a large amount of egg whites, this may take longer than you think, about 8 to 10 minutes. Do not overwhip -- the egg whites should look soft and smooth, not clumpy or dry.
Add the orange zest and vanilla bean seeds and beat just until combined.
Pour the meringue gently onto the prepared baking sheet and spread it into a circle, using the traced circle on the parchment as a guide.
Bake until the meringue is firm and set, 1 1/2 to 2 hours. Turn the oven off and leave the meringue inside to cool completely (and continue drying), another 3 to 4 hours.
When ready to serve, carefully remove the meringue from the parchment and transfer to a large plate or platter. In a small pot, bring the 1/2 cup of cream to a simmer. Pour the cream over the white chocolate, let sit for 15 seconds, then stir to combine. (If the mixture doesn’t melt completely, place the bowl over a double boiler and stir constantly until melted.) Gently pour the ganache onto the meringue and spread to the edges.
In the bowl of an electric mixer, beat the remaining cream on medium-high speed to medium peaks. Gradually add the powdered sugar and mix to combine. Spread the whipped cream on top of the ganache (which should have set by now), and top generously with orange zest. Serve by slicing like a pie or tart.
I always carry three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's pie. My first cookbook, The Fearless Baker, is out on October 24, 2017.