Preheat the oven to 400°F (204°C). Line a baking sheet with parchment paper or Silpat.
Pour the sugar into a small roasting pan and place in the preheated oven for about 7 minutes, or until the sugar is hot to the touch and just barely starting to melt at the edges. Remove from the oven and turn the oven down to 225°F (107°C).
While the sugar is in the oven, place the egg whites in the bowl of a stand mixer fitted with a whisk attachment. Add the salt and whisk until frothy.
Quickly pour all the hot sugar onto the egg whites in one go and continue to whisk on high speed for 8 to 10 minutes, until the meringue mixture is very stiff and white.
In a small bowl, mix together the cocoa and cinnamon. Pour the cocoa mixture into the meringue mixture and, using a large metal spoon, very lightly fold it in, using just two or three strokes. The trick is to keep the mixture quite marbled in appearance.
Spoon the mixture onto the prepared baking sheet in 4 or 6 equal-size peaky meringues. Cook on the middle rack of the preheated oven for 1 1/2 to 3 3/4 hours, until the meringues are dry and crisp throughout. (Yes, we know, that’s quite the range in time. It all depends on the size of your meringues and the humidity in your kichen.)
Remove from the oven and let cool on the baking sheet. (The cookies will keep for 3 to 4 days in an airtight container. Find more information on storing your cookies here.)