1. Start by preheat ting your oven to 180C/160C fan/gas 4 and line a 12-hole muffin tin with large cupcake cases.
2. In the bowl of your stand mixer, cream together your butter and sugar until light, fluffy and pale in colour.
3. In a separate jug, whisk your eggs and gradually add these to the butter/sugar mixture, mixing on a low speed until fully combined.
Top Tip: If the mixture looks like it may be splitting or curdling, add a heaped tablespoon of your flour to bind it back together.
4. Sieve the flour and cocoa powder into the bowl and mix until just combined – be careful not to over-beat the mixture.
5. Divide the mixture between your cases and bake at the top of the oven for 25-27 minutes (until the mixture stops ‘singing’).
6. Remove from the tin and place on a wire rack to cool completely.
7. While your cakes are cooling, make the buttercream. Begin by breaking up your chocolate into cubes and placing in a microwave-safe bowl.
8. Heat the chocolate in a microwave 30 seconds, before stirring. Continue to heat and stir in 30 second intervals until the chocolate is completely melted.
9. Set the chocolate aside to cool for five minutes while you cream your butter in the bowl of a stand mixer until pale and smooth. This may take a while, but the longer you leave it, the better.
10. Add in the icing sugar, a little at a time, ensuring each addition is fully incorporated before adding the next.
11. Finally, add the cocoa powder, the melted chocolate and the milk, and mix again until smooth.
12. Transfer your buttercream into a piping bag, fitted with a closed star tip and pipe a generous swirl of buttercream onto each cupcake, before topping with a scattering of rainbow sprinkles.