Preheat oven to 180 degrees celsius. Grease and line a loaf pan with baking paper and set aside
Mix the cocoa and water together into a paste and set aside
Beat the butter and sugar with an electric mixer,on low speed for about 2 minutes. Increase the speed to a medium setting and beat for a minute
Add the eggs, one at a time and beat until each one is combined. Continue beating for at least 4 minutes until light and fluffy
Mix in the vanilla essence
Add half the flour and baking powder and half the milk and beat, still using the electric mixer
Add the other half of the flour, baking powder and milk and mix until combined. Do not overmix
Place half the batter into another bowl and mix the cocoa paste into half the batter, using the electric mixer.
To create the marble effect, pour half a cup of the vanilla batter in the middle of the loaf pan. Do not spread. Place a little less than half cup of the chocolate batter in the middle of the vanilla and repeat this process until the batter is finished. Doesn't have to be perfect. The batter will remain in the centre and will start spreading as you add more batter. Tap the pan on the counter top a few times and the batter will spread. Bake at 180 degrees Celsius for 55-60 minutes or until a toothpick inserted comes out clean.
Once removed from oven allow it to cool for 1o minutes. Remove from pan and place on a cooling rack to cool completely before storing in an airtight container