Preheat the oven to 320 degrees F (160 degrees C).
In the bowl of a stand-mixer fitted with the whisk attachment, combine the Egg (4) and Caster Sugar (2 cup) and mix on a high speed for 3-5 minutes until light and fluffy.
Combine the All-Purpose Flour (2 cup), Dark Cocoa Powder (8 teaspoon), Baking Powder (4 teaspoon) and Salt (1 pinch) in a bowl and whisk together to get rid of any lumps.
Add the dry ingredients to the egg and sugar mixture and mix on a low speed until just combined.
Combine the Butter (1/2 cup) and Milk (1 cup) in a saucepan and melt the butter over a low heat. Bring to the boil and remove from the heat.
Add the boiled milk and butter mixture to the bowl of the stand mixer, and whisk on high until completely combined.
Pour the cake batter into an oven-proof dish and place in the oven. Bake for around 45-60 minutes.
Remove the cake from the oven and allow to cool completely. While the cake is cooling, make the frosting.
Place the Butter (1 cup) in the bowl of a stand mixer with the paddle attached.
Mix on a high speed until the butter becomes light and pale, around 5 minutes.
Add the Powdered Confectioners Sugar (2 cup) and Dark Cocoa Powder (1/2 cup), mix on a high speed until thick and combined.
Add the Milk (1/4 cup) and continue mixing until light and fluffy.
Frost the cake using an off-set spatula knife.
Decorate with Sprinkles (to taste). Serve and enjoy!