Preheat oven to 200 degrees with racks in upper and lower thirds. Line four baking sheets with parchment paper; set aside. Add egg whites, sugar, and cream of tartar to the bowl of an electric mixer; set over a saucepan of simmering water. Whisk constantly, until sugar is dissolved and egg whites are warm to the touch, about 3 minutes.Advertisement
Transfer bowl to mixer; fit with the whisk attachment. Beat on high until egg whites are cool and stiff peaks form, about 7 minutes. Divide meringue in half. Sift cocoa powder over one half; fold in gently.
Place meringue batches in separate piping bags, each fitted with a 1/2-inch plain tip. Pipe 3/4-inch-wide kisses onto prepared sheets. Bake in batches until hard to the touch but not browned, about 1 hour. Transfer to a wire rack; let cool completely.
Melt chocolate, stirring, in a heatproof bowl set over a pan of simmering water; remove from heat, and stir in oil. Dip bottoms of kisses in chocolate; transfer to four parchment-paper lined baking sheets to harden, about 1 hour.