Into a saucepan under low heat, add and mix the Caster Sugar (1/2 cup), Butter (150 gram), Baking Soda (1/2 teaspoon), Unsweetened Cocoa Powder (2 tablespoon), and Water (1 cup). Turn the heat up to high, and let the mixture come to a boil.
Once the mixture comes to a boil, transfer it to a separate bowl and allow it to cool for about 15 minutes.
Into the mixture, add in the Self-Rising Flour (1/2 cup), and Egg (2). Mix everything together.
Transfer the mixture to a baking tray lined with some baking paper. Bake under 180 degrees C (350 degrees F) for about 35 minutes.