Preheat oven to 200˚. Line a baking sheet (or two) with parchment paper and set aside.

In a large bowl, using an electric mixer with whisk attachment on medium-high, mix egg whites until frothy. Add the cream of tartar and salt and mix until egg whites hold at least soft peaks; about 1 to 2 minutes.

Add the sugar in 2-tablespoon increments, and continue to mix over medium-high speed until all sugar is incorporated and the mixture is glossy, thick, and can hold very stiff peaks, about 5 minutes more.

Add the vanilla and cocoa powder and mix on low speed until combined. With a spatula, gently fold in the chocolate chips by hand until just combined.

Fill a large plastic sandwich bag with the mixture and cut a small hole in the corner (alternatively you can use a pastry bag or drop heaping tablespoons). Pipe the mixture into 2-inch rounds as evenly as possible. If you mess up piping, you can spoon the mixture back into the bag, wipe the parchment, and try again.

Bake for 2 hours. Turn oven off, and let the cookies cool in the oven and dry out for another 2 hours.