Preheat the oven to 350F/ 180 C.
Take a mixing bowl and crack the eggs into it. Sprinkle the sugar and whisk to mix it together. Then, pour the oil in the bowl. Place a sifter on the bowl. Sift the flour, cocoa powder, baking powder, baking soda and salt into the bowl. Whisk it to form a smooth batter. Pour the hot water and whisk again.
Place cupcake liners on the muffin tin and pour the batter about half way through in each muffin hole.
Bake for 20 minutes in the middle rack of the oven.
Once baked, cool it down completely before you apply the frosting.
For the frosting, whisk the butter until smooth. Add the powdered sugar and cocoa powder. Whisk it again to form a crumbly texture. Pour the milk and keep whisking until it turns into a smooth frosting (If you feel it’s not smooth enough, add a little bit more milk).
Once the cupcakes are cooled down completely, apply the frosting either using a pipping bag or an icing spatula.
Sprinkle some rainbow sprinkles on top, if you wish.