Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 9x9 inch pans.
In a medium bowl, cream together the sugar and butter. Beat in the eggs, one at a time, then stir in the vanilla. Combine flour and baking powder, add to the creamed mixture and mix well. Finally stir in the milk , orange juice and orange zest until batter is smooth. Pour or spoon batter into the prepared pan.
Bake for 20 to 25 minutes in the preheated oven. Cake is done when it springs back to the touch.
Filling
Beat butter until fluffy. Add powdered sugar and orange zest. Slowly pour in orange juice while continuing to beat in. Add more powdered sugar or orange juice to get to desired consistency. Frost one layer of cake and add chocolate chunks if desired. Top with second layer.
Recipe by The Kitchen of Good and Evil at http://www.kitchenofgoodandevil.com/?p=242