Preheat oven to 250 degrees. Line a large baking sheet with parchment or a Silpat.
2
In a small bowl, combine cocoa and PB2. Set aside.
3
In a large bowl, beat egg whites until frothy using an electric mixer set to medium speed. Add cream of tartar, salt, and vanilla; increase speed to high, and continue beating until soft peaks form. Slowly add sugar, 1 T at a time, beating until stiff peaks form and mixture becomes glossy. Gently fold in cocoa mixture until combined.
4
Using a rubber spatula, transfer mixture to a pastry bag fitted with a large star tip. (Or cut 1/4 inch off the corner of a large plastic bag.) Pipe small mounds onto prepared baking sheets.
5
Bake for about 45 minutes. Turn oven off and leave cookies in for at least 45 minutes more. Remove from oven; transfer to a wire rack and cool completely.
