Melt 4 ounces finely chopped bittersweet chocolate in a double boiler or a heatproof bowl placed on top of a pot of boiling water. Stir occasionally with a rubber spatula until melted completely, before setting aside to cool completely on a wire rack.
Preheat the oven to 275 (F). Prepare a baking sheet by lining with parchment paper or a Silpat mat.
Place 3 large egg whites in the bowl of a freestanding electric mixer fitted with a whisk attachment. Whisk on medium speed until soft peaks form.
While the egg whites are whisking, whisk together 3/4 cup granulated sugar and 2 tablespoons confectioner's sugar in a small bowl. Once soft peaks have formed, slowly stream the sugar into the egg whites, before adding 3/4 teaspoon orange blossom water and 1/2 teaspoon white vinegar.
Once all the ingredients have been incorporated, increase the mixer speed to medium-high and continue whisking until the meringue is glossy and holds stiff peaks.
Once the meringue is stiff, drizzle a some cooled chocolate (from the 1st step) on top of the meringue – DO NOT MIX IN. Unmixed chocolate drizzle is what gives the meringues their interesting swirls. Use a 3-tablespoon sized cookie dough scoop to portion out a meringue onto the prepared baking tin. Continue drizzling chocolate over the meringues before scooping until all the meringue batter is used.
Bake meringues in the preheated oven for about 50 - 60 minutes, or until the bottom of each meringue peels easily away from the parchment paper. Remove tin from oven and place on a wire rack, allowing the meringues to cool overnight uncovered on the wire rack in a dark, dry place.