Heat the oven to 200 degrees F and line two baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the whisk attachment, whip up the egg whites with the salt and cream of tartar until stiff peaks begin to form (this can take up to 10 minutes). This is important – do not rush this! I literally have to leave the kitchen and occupy myself with something else so that I don’t overthink this part and stop it too early. Do some dishes. Play a phone game. Cuddle a furbaby. Seriously, set a timer and walk away (just make sure your mixer is safely situated to leave unattended).
When stiff peaks have begun to form, reduce the speed and gently add one tablespoon of sugar to the mixture. Whip this back up to stiff peaks. Gradually repeat this process with the remaining sugar, adding one or two tablespoons of sugar at a time until the peaks are nice and glossy. You may need to stop once or twice in this process to scrap up any unincorporated sugar from the bottom of the bowl with a spatula.
Whip in the espresso powder, and then fold in the chocolate with the spatula.
Transfer the meringue mixture to a pastry bag and pipe out the meringue on the prepared baking sheets. Bake in the oven for 1 hour. Turn off the heat, leave the oven door closed, and let the meringue sit as the oven cools for 2-3 hours (or overnight).
Store in an airtight container at room temperature for up to 3 days.