In the heatproof bowl of a stand mixer, whisk together the egg whites and sugar. Set over a saucepan of simmering water, making sure the bottom of the bowl doesn’t touch the water. Whisk frequently until sugar is dissolved. I tested with my fingers to make sure all sugar granules were dissolved.

Return the bowl to the stand mixer and beat on high speed with the whisk attachment until meringue is stiff and glossy, about 5 minutes. Sift the cocoa powder over the meringue, add the chopped chocolate, and gently fold together with a spatula until incorporated.

Scoop out onto the prepared sheet pans in rounded tablespoons, about 2 inches apart. Baking 2 sheet pans at a time, bake for 8 minutes then rotate pans and bake for an additional 8 minutes, until meringues are puffed and cracked. Let cool completely.

Store in an airtight container at room temperature.