Pre-heat the oven to 325°F.
Grease and flour a bundt cake pan; set aside.
Using a medium bowl and an electric mixer, cream the butter, sugar, vanilla and orange zest well.
Add the eggs, one at a time, beating well after each addition.
In a medium bowl, sift together the all-purpose flour, cake flour and baking powder.
Fold the dry ingredients into the creamed mixture, in 3 portions, alternating with 2 portions of milk; begin and end with the dry ingredients.
Pour batter into the well greased and floured bundt pan.
Place in the pre-heated oven and bake for about 40 - 45 minutes or until a toothpick inserted in the center comes out clean; baking times vary depending upon the size of your bundt pan so the toothpick test is the best method to ensure that the cake is completely baked.
Remove from the oven and cool completely on a wire rack.