How to Make It
Preheat the oven to 225°. Line a large rimmed baking sheet with parchment paper. In a medium bowl, using a hand mixer, beat the egg whites with the cream of tartar at low speed until foamy, about 30 seconds. Increase the speed to medium and beat in the sugar, 1 tablespoon at a time; beat until the whites are stiff and glossy, about 1 minute. Fold in the chopped chocolate and a pinch of salt. Sift the cocoa powder and espresso powder over the meringue and fold 2 or 3 times to incorporate; the meringue should look marbled.
Spoon eight 1/2-cup mounds of meringue onto the prepared baking sheet. Using the back of a spoon, gently spread the meringues into 3-inch rounds. Bake for 1 hour, or until the meringues are firm on the outside but still chewy in the center. Let cool completely.