Preheat the oven to 170°C /340F°F. Line a 9x13inch rectangular with baking paper.
Mix the milk and lemon juice and leave to stand for 5 minutes or use buttermilk. 

Put the flour, cocoa powder and sugar in large bowl. Add the baking powder, bicarbonate of soda and salt and mix together using a balloon whisk.
Add the butter, eggs and buttermilk / soured milk.

Increase the speed gradually, until the batter is completely smooth. Stop the mixer and scrape the sides and bottom of the bowl to make sure everything is incorporated.

Transfer the batter in the prepared tin and level.

Transfer the batter into the prepared cake tin and bake for 35 -40 minutes.

Check the cake is done by inserting a skewer in the centre – it should come out clean, the cake should be well risen. Cool the cake before frosting.

🌻Chocolate Frosting:
Put the chocolate in a bowl and melt in the microwave, in short 30 second bursts, until it melts.
Or use the double boiler method.
Leave the chocolate to cool slightly.

Put the icing sugar and cocoa powder in a mixing bowl (or the bowl of your stand mixer) and mix together to combine. 

Add the softened butter and start mixing on low speed, gradually adding the milk until the frosting is well combined.

Add the melted and cooled chocolate and beat together until the frosting holds soft peaks.

🌻Assemble the Cake:
Use a palette knife to spread the frosting over the cake. 

Add sprinkles or leave plain. Slice the cake using a serrated knife, wiping the knife clean between each slice.