Preheat the oven to 350°F (175°C). Lightly grease a 10-inch square pan.
In a large bowl, use a whisk or an electric mixer to cream the butter for 3-4 minutes or until it becomes fluffy. Then, add the caster sugar and continue mixing for another 3-4 minutes until it's fluffy as well.
Add the eggs to the bowl and whisk until they are well combined. Mix in the vanilla extract until fully incorporated.
Sift the plain flour, baking powder, salt, and cocoa powder into the wet batter. Then, use a spatula or wooden spoon to gently fold in the flour until everything is well combined.
Gradually pour in the milk in stages, stirring to achieve a smooth, lump-free consistency. The batter should be smooth, fluffy, and of a medium thickness
Pour the batter into the prepared square pan and set it aside.
In a separate bowl, combine the sugar and cocoa powder, ensuring a thorough mix, and set it aside. Sprinkle chocolate chips evenly over the batter in the pan, and then sprinkle the sugar and cocoa powder mixture on top.
Meanwhile, boil water in a kettle, and once it's boiled, carefully pour the hot water around the sides of the cake until it is fully covered.
Bake the cake in the preheated oven for 35-40 minutes or until the centre is almost fully cooked. Allow it to cool for 15-20 minutes before serving. Enjoy