Preheat your oven to 325 degrees F. Grease two large baking sheets with baking spray.
Separate your egg while it's cold, placing the egg white in a medium bowl. (Reserve the yolk in a separate container, cover it and refrigerate for another use.)
Coarsely chop your pecans and set them aside.
Add your cream of tartar and salt to the egg white. Mix with an electric hand mixer, using a whisk attachment, if you have one. (A stand mixer doesn't work well for such a small amount.) Mix just until it is very frothy and starting to come together.
Gradually add your brown sugar, mixing after each addition and scraping down the sides of your bowl as needed. (It helps to place your bowl on top of a kitchen towel to prevent sliding.)
Add your vanilla and ½ teaspoon cocoa. Beat again on high speed, until the mixture is thick and stiff, similar to marshmallow fluff. (See the photo in the post.) It's okay if the meringue peaks fall over and aren't super stiff; mine never reach that stage.)
Stir your chopped pecans into the meringue. Using a scoop or two teaspoons (one to scoop, one to scrape), place walnut-sized mounds of meringue onto your pans, at least 1 ½ inches apart. You should be able to fit a dozen on one large sheet pan.
Bake the cookies for 15 minutes, or just until they are puffy and set. Turn off the heat and let the cookies stay in the oven for another two minutes to dry.
Remove the cookies from the oven and sprinkle some cocoa on top of them by placing a bit of cocoa in a fine-mesh strainer and gently shaking it over the cookies. While the cookies are still warm, carefully transfer them to a wire cooling rack.
Store the cooled-off cookies at room temperature, laying them flat in a single layer in a plastic container, then placing wax paper on top before starting the next layer. They should stay good for three days, but they're usually gone before that!