Use this recipe for our Individual Chocolate-Espresso Charlottes.
Process butter and confectioners' sugar in a food processor until fluffy. While machine is running, gradually add egg whites and vanilla, and scrape down sides of bowl. Add flour, and process until combined.
Transfer to a bowl, and stir in cocoa powder with a rubber spatula. For piping, transfer to a pastry bag fitted with a 1/16-inch plain round tip (such as Ateco #2).