Preheat the oven to 325 degrees F (160 degrees C).
In the bowl of a stand-mixer fitted with the whisk attachment, combine the Egg (4) and Caster Sugar (360 gram) and mix on a high speed for 3-5 minutes until light and fluffy.
Combine the All-Purpose Flour (240 gram), Dark Cocoa Powder (2 tablespoon) (plus 2 tsp), Baking Powder (4 teaspoon), and Salt (1 pinch) in a bowl and whisk together to get rid of any lumps.
Combine the Butter (125 gram) and Milk (240 milliliter) in a saucepan and melt the butter over low heat. Bring to a boil, then remove from the heat.
Add the flour mixture and the boiled milk and butter mixture to the bowl of the stand mixer. Whisk on high until completely combined.
Spoon the mixture into your cupcakes liners and place in the oven. Bake for around 30 to 40 minutes.
Remove the cupcakes from the oven and allow to cool completely. While the cake is cooling, make the frosting.
Place the Butter (250 gram) 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 (240 gram) and Dark Cocoa Powder (8 tablespoon). Mix on high speed until thick and combined.
Add the Milk (60 milliliter) and continue mixing until light and fluffy.
Top each cupcake with frosting and decorate with Sprinkles (to taste).