Heat oven to 350. Lightly spray a bundt pan w/ cooking spray. In lg bowl, w/ mixer on medium speed, cream butter, sugar and orange zest until fluffy.
Add eggs and beat well. In another lg bowl, combine flours, and 1 tsp baking soda. Add flour mix, alternateky w/ sour cream to butter mix. Beat until well mixed.
In another bowl, combine chocolate syrup and 1/4 tsp baking soda. Add 2 cups batter to chocolate mix, mix well. Pour remaining batter into pan. Pour chocolate batter over yellow batter, don't mix. Bake about 65 mins, or until a toothpick inserted in center comes out clean. Sprinkle cooled cake w/ powdered sugar.