To make the Pavlova: Preheat the oven to 250 degrees F. Line a baking sheet with parchment.
To the bowl of a stand-up mixer, with the whisk attachment, add the egg whites and vinegar; beat until frothy, about 2 minutes. With the mixer running, slowly incorporate the sugar; beat until stiff peaks form, about 5 additional minutes. Mix in the vanilla extract. Next, sift the cocoa powder atop the meringue; fold in the powder using a spatula. When the cocoa powder is thoroughly incorporated, fold in the dark chocolate.
Take a 5 or 6-inch plate, flip it over onto the parchment, and use it trace 3 circles. Divide the meringue evenly amongst circles, smoothing them out until they’re about 1-inch thick. Transfer to the oven to bake for 1 hour until hard and crisp on the outside. Turn the oven off, prop open the oven door, and allow to cool completely on the baking sheet.
To make the 7-Minute Frosting: In large metal bowl, whisk together the water, sugar, egg whites, cream of tartar, and salt. Set bowl over pan of barely simmering water and mix with handheld electric mixer at low speed. Gradually increase speed to high, beating until mixture holds stiff peaks, about 5 minutes.
Transfer bowl from pan to folded kitchen towel on counter and continue beating until mixture is cool and billowy, about 2 minutes more. Beat in vanilla. (Frosting can be made 4 hours ahead and chilled, covered.)
To assemble: Place one chocolate meringue on a plate. Top with 1/3 of the 7-minute frosting and smooth it out. Using a torch, toast the frosting until golden brown. Sprinkle with about a tablespoon of graham cracker crumbs. Repeat with the remaining meringue, frosting and graham crackers. Pavlova stays good in the fridge for up to 1 day. If you like, you can make the meringue up to 3 days ahead and assemble the day of.