Preheat oven to 275 degrees. Line two baking sheets with parchment paper. Set aside.
In a small bowl, combine the hazelnut meal and the cornstarch. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment, add the egg whites and salt. Beat on medium speed until foamy, then slowly add the sugar, vanilla, and vinegar. Increase speed to medium-high and beat until glossy and the meringue holds stiff peaks. This will take several minutes.
Remove the bowl from the mixer. Add the hazelnut-cornstarch mixture and fold in (gently) by hand with a large spatula.
Spoon or pipe meringue mixture onto the prepared baking sheets in even-sized circles, about 1½ inches in diameter, making sure to keep the meringues relatively flat. You may use a dampened fingertip to gently pat down any peaks.
Bake for 45 minutes, then turn off the oven. Crack the door slightly and allow the oven to cool with the meringues inside.
To assemble the cakes: Place a metal bowl and the electric mixer beaters in the freezer for 10-15 minutes or the fridge for 20-30 minutes. Add the powdered sugar, cream, and vanilla to the cold bowl and beat until stiff peaks form.
Spread half of the meringues with a small amount of the blackberry preserves. Spread a small amount of whipped cream on top of the preserves. Top with a second meringue; spread a bit more whipped cream on top of the “cake” and garnish with a fresh blackberry. Cakes can be assembled ahead of time and stored in the refrigerator, but meringues will soften a bit if assembled far in advance; assemble within an hour of serving if you want the meringues to stay crunchy.