We take the guesswork out of making caramel with our foolproof microwave method. After it cools, chop and sprinkle over the cake for a sweet crunch.
Meringue: Preheat oven to 275 degrees. Butter three 8-inch round cake pans. Line bottoms and sides with parchment, leaving at least 1/2 inch extra on sides; butter parchment. Pulse sugar, cornstarch, and salt in a food processor until finely ground.
With a mixer on medium-high speed, beat egg whites and cream of tartar until soft peaks form. Beat in sugar mixture, 1 tablespoon at a time, then beat until stiff, glossy peaks form, about 5 minutes more. Divide mixture among pans; smooth tops. Bake 1 hour, then turn off oven and prop open door. Let cool 1 hour. Transfer pan to a wire rack; let cool completely.
Cream: In a bowl, whisk together sugar, flour, and salt. In a saucepan, combine buttermilk, yolks, and vanilla pod and seeds; place over medium heat. Gradually whisk in sugar mixture. Cook, stirring, until just boiling, about 9 minutes. Strain through a fine-mesh sieve, pressing on solids; press plastic wrap directly on surface and refrigerate until cold, at least 1 hour and up to 3 days.
Caramel: Coat a baking sheet with cooking spray. Stir together sugar and corn syrup in a microwave-proof bowl. Microwave on high 3 minutes, stirring after each minute. Stir again, then microwave just until golden and caramelized, about 15 seconds more. Immediately pour mixture onto prepared sheet and let cool until firm. Remove from sheet; finely chop.
Using parchment, lift meringue cakes from pans; discard parchment. Carefully place one cake on a platter or cake stand. Spread half of cream on top. Sprinkle with 3 tablespoons caramel. Repeat with second cake layer, remaining cream, and 3 more tablespoons caramel. Top with whipped cream and remaining caramel; serve immediately.