Grease a loaf pan 10 inch *4inch *2inch with butter and dust a bit of flour at the bottom. I use a littler form my sieved flour mix only.
Cream the butter and sugar together till it gets a pale white color.
Add eggs gradually and keep beating with an electric blender till well blended. Add vanilla essence and mix well.
Now, add the orange juice and butter milk and beat well. Add the orange zest and just mix with a spatula now.
In another bowl, sieve together whole-wheat flour, all-purpose flour, cocoa powder, baking powder and baking soda.
Now,put 1/3rd flour mix to the liquid mix/butter n egg mix and with a spatula softly with cut and fold method.
Keep repeating till the flour mix is mixed completely.
Pour the batter into the mould and bake for 20 minutes.Ater 20 minutes reduce the oven temp. to 170 degrees and bake for another 20 minutes or till done.Check with a toothpick inserted and came out clean.
Enjoy your healthy wholewheat chocolate buttermilk cake.