Preheat the oven to 180°C. Mix the cocoa and bicarbonate of soda with the boiling water and leave to cool.
In a large bowl, whisk the eggs and sugar together. Add the oil and whisk again.
Then add the flour and cocoa, mixing thoroughly after each addition. Pour into cupcake cases and cook for 20-25 minutes, until the cakes are firm to the touch and a skewer or sharp knife comes out clean.
To make the frosting:
Cream the butter in an upright mixer or hand mixer and beat until soft.
Add the cream cheese in 2 or three stages followed by the icing sugar again in 2 or 3 stages.
Add the lemon juice and beat for a few minutes.
Leave to stand in a cool place for at least an hour.
Take 500g of the frosting and add 2 tbsp of cocoa powder.
In a piping bag fitted with a star tube place the chocolate frosting down one side of the piping bag and half the remaining frosting down the other half of the bag. Pipe swirls onto each cupcake.
Note: It is very important that the butter is very soft before you add the cream cheese or it will split and make for an unpleasant consistency. With these quantities there will be a about 200g of frosting left over.