Preheat the oven to 225°F and line two baking sheets with parchment paper.

In a medium mixing bowl, beat the egg whites and the cream of tartar using an electric mixer on low speed until foamy. Increase the speed to medium and begin adding the sugar, a tablespoon at a time, and beat until the meringue forms stiff and glossy peaks. Fold in the chocolate and salt, and then sift the cocoa and espresso powder over the meringue. Using a large rubber spatula, fold the mixture onto itself several times to incorporate the powders.

Spoon the meringue into a pastry bag fitted with a large plain tip, and pipe the mixture onto the lined baking sheets, forming 1-inch size kisses and staggering rows of the kisses about 1 1/2 inches apart. Bake in the center of the oven until the kisses are crisp on the outside and yet still slightly chewy inside, about 45 minutes. Remove from the oven and set aside until completely cool. Serve or store in an airtight container for up to 1 week.