1. Preheat your oven to 180°C/160°C/gas mark 4 and grease and line a 9” by 13” traybake tin.
2. In a large bowl, cream together your butter and sugar until light, fluffy and pale in colour.
3. In a separate jug, whisk your eggs and gradually add them to the butter/sugar mixture along with the vanilla extract, mixing until fully incorporated.
4. Sieve the flour, baking powder and cocoa powder into the bowl and mix until just combined.
5. Spoon the mixture into the tin and use a spatula to spread it out evenly.
6. Bake your traybake in the oven for 45-50 mins, until a thin skewer inserted into the centre of the cake comes out clean.
7. Once the cake is baked, remove it from the oven and allow it to cool in the tin until room temperature.
8. While your cake is cooling, make the buttercream. Start by breaking up the chocolate into segments and place in a microwave-safe bowl.
9. Heat the chocolate in a microwave for 30 seconds, before stirring. Continue to heat and stir in 15 second intervals until the chocolate is completely melted.
10. Set the chocolate aside to cool and place the butter in the bowl of a stand mixer. Use the beater attachment to whip it until it is smooth and very pale. This may take a while, but the longer you leave it, the better.
11. Add the icing sugar, a little at a time, ensuring each addition is fully incorporated before adding the next.
12. Finally, add the cocoa powder, the melted chocolate and the milk, and mix again until smooth.
13. Cover the bowl with cling film until you are ready to use it.
14. When you are ready to assemble your traybake, begin by transferring the cake onto a large plate or cake board.
11. Give the buttercream a final mix, before adding to the top of the cake and spreading out evenly with an offset spatula.
12. Decorate the cake with plenty of sprinkles and slice into small square-shaped portions to serve.