Preheat the oven to 180 degrees celsius. Grease and line a loaf pan, 23cmx13cm and set aside
Sift the flour and bicarbonate of soda and set aside
Using an electric mixer, beat the butter and sugar until light and creamy, for a low speed at first and then on medium speed, or at least 4 minutes
Beat in the eggs, one at a time, until well incorporated
Mix in the orange zest and orange juice
Add the flour mixture in thirds together with a third of the milk. Mix until combined, on a low speed
Place half the batter in a separate bowl and mix in the cocoa powder. Do not over mix
To create a marble effect, add a dollop of chocolate batter at the bottom centre of the loaf pan. Add a dollop of the plain batter on top and repeat this process. Tap the pan gently on a flat surface
Bake for 45-55 minutes or until a toothpick inserted comes out clean
Allow the cake to cool for at least 10 minutes before removing it from the pan. Place on a wire rack and allow it to cool completely before cutting