Preheat your oven to 160C / 320F (fan off), 140C / 285F (fan on). Line your muffin tin with cupcake liners. More details on cupcake liners in the notes.

Combine dry ingredients - Add the flour, sugar, cocoa powder, bicarb soda, baking powder and salt to a medium sized mixing bowl and sieve. Then mix with a hand whisk.

Add wet ingredients - Add the butter, oil, eggs and milk to the dry ingredients and whisk together until well combined and no dry ingredients are showing. Take care not to overmix the batter.

Add boiling water - Lastly, pour in the boiling water and whisk until well combined and smooth. The batter will darken and thin out.

Bake - Scoop the batter into the cupcakes filling them up ¾ of the way. Bake in the middle rack of your oven for 30-35 minutes or until a toothpick inserted into the center of one cupcake (one of the middle cupcakes) comes out clean. Transfer to a cooling rack to cool to room temperature.

Cream butter - Add your softened butter, salt and vanilla extract to a large bowl. If using a stand mixer fit it with the paddle attachment. Beat on medium high speed for 5 minutes until pale and fluffy. You will want to scrape down the bowl halfway through with a spatula to ensure everything beats together evenly!

Add sugar - Scrape the bowl down using a spatula and add all of the sifted powdered sugar. Begin beating on low speed at first. When no dry ingredients are showing, turn the speed up to medium high speed again and beat for 5 minutes.

Add chocolate and cream - Scrape down the bowl again and add the melted chocolate and heavy cream. Beat in on medium high speed for 2 minutes. Scrape down the bowl at least once to ensure the chocolate mixes in evenly.

Add the frosting to a piping bag fitted with an open star top and pipe swirls on each cupcake. Finish with chocolate or rainbow sprinkles.