Beat egg white and cream of tartar with a mixer at high speed until soft peaks form. Gradually add sugar, then vanilla, then cocoa and coffee powder. Drop meringue onto foil lined baking sheet in 12 mounds about 2 in. (5 cm) apart. Bake 40 minutes at 250F (120C) or until firm. Turn off oven and let meringues cool in oven for 1 hour. Do not open oven door.