Combine eggs, orange juice, oil, vanilla and sugar in a large mixing bowl. Slowly add flour, baking powder, baking soda and orange rind. Mix until well combined. Pour most of cake batter into a greased bundt or spring pan, leaving about a cup of batter in your bowl.
In your bowl, add cocoa powder and sugar (or chocolate milk powder) to batter and stir well. Pour your chocolate batter over the other batter. Gently cut a knife through the cake to get a nice marble effect.
Bake at 350 degrees for approximately 30 minutes. (use the toothpick test!).