Preheat to 350 degrees. Generously spray a bundt pan with baking spray.
With an an electric mixer, combine the flour, sugar, baking soda, salt, cinnamon and cocoa. Stir in the orange juice, mayonnaise, vinegar and vanilla and mix until just combined. Fold in the orange zest.
Pour the batter into the prepared pan and bake until a toothpick inserted in the cake comes out clean, 45-55 minutes. Cool in the pan for 10 minutes, then turn out onto a cooling rack to cool completely.
Meanwhile, make the glaze by stirring the powdered sugar and cocoa together in a medium bowl. Whisk in the orange juice, 1 tablespoon at a time, until glaze reaches a good consistency for drizzling. Spoon the glaze over the cooled cake.