Heat oven to 375°F. Spray a 9-inch cake pan with nonstick cooking spray.
In a large bowl, whisk together flour, sugar, baking powder and salt. Stir in the orange zest.
Make a well in the flour mixture. Pour in the milk, orange juice, corn oil, egg and vanilla. Stir until just combined and all dry ingredients are moistened. Turn into prepared pan.
In a small bowl, mix together sugar, flour and butter until the consistency of cornmeal. Sprinkle mixture evenly over the cake batter.
Bake at 375°F for 40 to 45 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely in pan. Dust with confectioners' sugar, if desired, before slicing and serving.