Preheat oven to 250 degrees. Line two baking sheets with parchment paper or use silicone mats. In a small bowl, sift together the cocoa powder and sugar. In a large bowl, beat egg whites, cream of tartar and salt with an electric mixer until soft peaks form. Add the cocoa mixture one tablespoon at a time, mixing between additions. Add peppermint extract and mix well. Drop by rounded tablespoonful on to baking sheets. Place in the oven for 1 hour. Turn off oven and leave door slightly open. Leave meringues for at least 2 hours or overnight. Store in an airtight container.