Preheat oven to 300 degrees and line a rimmed baking sheet with parchment.
Mix whites, sugars, and salt in a mixer bowl and whisk on medium-high speed until stiff peaks form, about 8 minutes. Beat in vanilla.
Sift cocoa powder over meringue, and fold until barely any streaks remain. Using the spatula spread meringue into a round on the sheet.
Bake meringue until dry to the touch, about 1 hour. Let cool on sheet on wire rack and then top as desired.