1/2 cup butter
3/4 cup sugar
2 eggs (you can increase this to 3 as well; I fell short of 1 egg but feel it will taste even better with an additional egg)
2 tsp orange essence (you can also substitute with vanilla essence)
100 ml orange juice (fresh is best, but canned will work as well)
1 tsp baking powder
1/2 tsp baking soda
50 ml milk
2 cups flour
1 1/2 tbsp cocoa powder
1 1/2 tbsp orange zest (peel the skin of oranges with a peeler and cut it very fine; you can also use candied orange peel chopped very fine or dried orange peels)
If using fresh orange, peel and cut the zest and keep aside and juice the orange.
In a bowl, cream the butter and sugar together. Add the eggs one at a time and whip till smooth and creamy. Add the orange/vanilla essence (I strongly recommend using orange) and the orange juice. Whip further for a minute. Now add the flour mixed with the baking powder and baking soda alternating with the milk. Stir well till no lumps remain. Now divide the batter into two equal or unequal portions and mix in the cocoa powder into one (I mixed into the one with more quantity) and orange zest into the other. In case you do not have access to orange zest (either fresh or candied or dried) you can skip it. However, you must add the orange essence then in the first step to achieve the perfect orangey flavor.
Pre-heat the oven at 180 degree celcius for 10 minutes. Now take a 8" round pan and spray with cooking spray. Line it with parchment paper and spray again. This helps in easy release of the baked cake. Now alternately spread the two batters in whatever pattern you wish to. With the help of a long skewer or toothpick, make swirls in the batter to help form a pattern inside.