Special equipment: One piping bag fitted with a plain 1/2-inch tip
For the meringue: Place an oven rack in the center of the oven and preheat to 200 degrees F. Line a baking sheet with parchment paper. Using a pencil, trace two 8-inch-diameter circles, side-by-side, on the parchment paper.
Beat the egg whites until fluffy in a stand mixer with a whisk attachment. Gradually add the superfine sugar, about 1 tablespoon at a time, with the machine running on medium-low speed, until the mixture holds soft peaks. Beat in the cream of tartar and salt. Increase the speed to high and beat until the mixture is thick and holds stiff peaks, 3 to 5 minutes. Beat in the vanilla extract. Place the meringue in the piping bag. Pipe the meringue into two 8-inch circles on the prepared baking sheet using the traced circles as a guide. Bake until crisp, 2 hours. Turn off the oven and allow the meringues to cool completely while still in the oven, about 2 hours.
For the cream: Beat the cream on high speed until thick using an electric hand mixer. Add the powdered sugar and vanilla extract. Continue to beat on high speed until the cream holds stiff peaks. Add the raspberries and beat until incorporated into the cream.
Spread one-third of the cream over each meringue circle. Place 1 circle on top of the other to create 2 layers. Spread the remaining cream on the sides of the cake. Garnish with fresh raspberries if using, and freeze the cake until firm, 3 hours.
For the syrup: Mix the raspberries, vinegar and maple syrup in a small saucepan. Lightly mash the raspberries using a fork. Bring the mixture to a boil over medium heat. Reduce the heat to a simmer and cook until thick, 25 to 30 minutes. Strain the mixture through a fine sieve and allow to cool.
To serve: Allow the cake to sit at room temperature for 15 minutes. Cut the cake into wedges and place on serving plates. Drizzle with the syrup and serve.