Preheat oven to 200°F (93°C). Cover several baking sheets with parchment paper.
To melt the chocolate, in a medium, heat-safe bowl, add the chocolate and set over a medium saucepan of simmering water. Do not let the bowl touch the water or let the water come to a boil. Stir frequently until melted and smooth. Set aside, stirring frequently while it cools.
Zest one orange, taking care to just get the orange part of the peel and none of the white pith. Set aside.
To make the meringues, in an electric stand mixer fitted with the whisk attachment, add the egg whites, cream of tartar and salt. Make sure the bowl and whisk are freshly cleaned and dried. Whisk on medium until the eggs are foamy and frothy (about 1-2 minutes) and then slowly add in the sugar a little bit at a time. You want it to slowly absorb into the egg whites and build structure. Once all of the sugar is in the mixture, turn the mixer on high and just before you reach stiff peaks, add the powdered sugar. Run the mixer on low to combine and then increase to high until you reach stiff peaks, which should take less than a minute depending on the speed of your mixer. Add the orange blossom water (if using) and run the machine for 30 seconds to fully incorporate. The meringue will look glossy and hold its shape when you invert the whisk. The tip of the inverted meringue will be just shy of 12 o’clock. It should also feel smooth when you rub some between your fingers. (See more Meringue Tips in the Before We Get Started section above the photos).
Using a 2-tablespoon cookie scoop or two spoons, drop 24 mounds of meringue on the prepared baking sheet. Drop about 1-2 teaspoons of chocolate on top and sprinkle with a pinch of orange zest. Use a toothpick or knife to swirl circles at the tops of the meringues...don’t dip down too deeply into the meringue; keep it up at the surface. Also, don’t do too many swirls, or you’ll lose some detail and it will just look messy.
Bake in the center of the oven for 2 hours. Check at some point between 1 and 1 1⁄2 hours to make sure the meringues aren’t browning and are still white. If they’re browning, turn off the oven. Let the meringues cool completely inside the oven. This should take several hours.
Meringues are best within several days of making, stored airtight at room temperature. If it is especially humid out allow extra time to dry out in the oven.