Mix all dry ingredients together EXCEPT splenda or sugar, set aside.
Mix all milk and all orange juice together, set aside.
Beat butter until light and fluffy, add splenda or sugar and beat until extra light and fluffy (about 2 mins).
Add eggs one at a time and beat after each addition.
Add vanilla and zest, and beat well to combine.
Fold in half flour mixture, mix slightly, fold in half milk/OJ mixture, mix slightly and repeat flour/OJ mixture combination. Fold until all is well combined.
Divide into 12 muffin cups (buttered if not using non-stick or silicone), sprinkle brown sugar over the top of each muffin (if using), and bake at 375 for 25-30 minutes or until cake tester comes out clean.