In a bowl, combine the flour, sugar, cocoa, baking soda and salt. In another bowl, combine the mayonnaise, orange peel and vanilla; gradually add orange juice until blended. Stir into dry ingredients just until combined. Stir in chocolate chips (batter will be thick). Fill paper-lined muffin cups two-thirds full.
Bake at 350° for 18-23 minutes or until a toothpick comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely. Dust with confectioners' sugar.