Preheat the oven to 375 degrees F (190 degrees C).
In a large bowl, cream together the butter, shortening and brown sugar until smooth. Beat in the egg and vanilla.
Combine the flour, baking soda, salt and cocoa; gradually stir into the creamed mixture. Drop by rounded spoonfuls onto ungreased cookie sheets.
Bake for 8 to 10 minutes in the preheated oven. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely. When cookies are cool, dust with confectioners' sugar.
