Mix together flour, sugar, baking powder add eggs then oil.
Divide the mixture and put one third to one side. Add the orange juice and zest to the bigger portion. If it is very stiff add 1 tablespoon of the milk.
add cocoa and the remaining milk to the third you had set aside.
Put the orange mixture in the tin first first, then add the chocolate mixture in a line along the middle of the orange mixture. Using the end of a teaspoon to gently swirl the chocolate mixture with the orange mixture.
Bake in the preheated oven until a tester comes out clean, about 40 minutes.