How to Make It
Preheat oven to 300°F with oven rack in lower third of oven. Combine egg whites and sugar in bowl of a heavy-duty stand mixer. Bring 1 1/2 inches of water to a simmer in a small saucepan over medium. Place bowl with egg white mixture over simmering water, ensuring base of bowl does not touch water. Cook, whisking constantly, until sugar is dissolved, 6 to 8 minutes.
Transfer bowl with egg white mixture to stand mixer fitted with a whisk attachment. Beat on medium-low speed, gradually increasing speed to high, until mixture is very thick, fluffy, and reaches room temperature, 6 to 8 minutes. (Mixture will resemble marshmallow fluff.) Using a rubber spatula, fold in chocolate, cacao nibs, and cocoa until incorporated.
Using a 1 3/4-inch scoop, drop small mounds (about 2 tablespoons each) onto 2 parchment paper–lined baking sheets, leaving 2 inches between mounds. Sprinkle mounds with cacao nibs. Bake in preheated oven until exteriors of meringues are dry and cookies can be lifted off of paper without sticking, 18 to 22 minutes, rotating baking sheets halfway through baking time. Let meringues cool completely on baking sheets, about 20 minutes. Store in an airtight container up to 1 week.