1. Preheat the oven to 225°F (110°C). Line two baking sheets with parchment paper.

2. In a large bowl, beat the egg whites with a hand mixer or stand mixer fitted with the whisk attachment until they form soft peaks.

3. Gradually add the sugar, 1 tablespoon at a time, while continuing to beat the egg whites until they form stiff, glossy peaks.

4. Gently fold in the cocoa powder and vanilla extract until just combined.

5. Scoop the meringue onto the prepared baking sheets, forming cookies about 2 inches wide and 1 inch tall.

6. Bake for 1 hour and 15 minutes, or until the cookies are dry and crisp on the outside.

7. Turn off the oven and leave the cookies inside with the door closed for an additional 30 minutes to allow them to cool gradually.

8. Remove the cookies from the oven and let them cool completely on the baking sheets before serving.