To make the sponges, add the butter and sugar to a bowl, and use an electric mixer to beat them well until creamy.
Add the eggs one by one, beating well after each addition.
In go the flour, cocoa powder and vanilla extract.
Beat again to incorporate.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Grease and flour a 12-hole muffin tray, and divide the batter evenly.
Bake for 20 minutes.
Remove from the oven, and leave to cool for 5 minutes.
Use a sharp knife to remove the cupcakes from the tray, transfer them to a cooling rack, and leave to cool completely.
To make the icing, add the butter, sifted icing sugar, milk and vanilla extract to a food processor and blitz until smooth.
Remove half of the icing, and set it aside in a bowl.
Add the cocoa powder to the remaining icing in the food processor, and blitz again.
Fit a nozzle onto a piping bag, and alternate one spoonful of vanilla buttercream, and one spoonful of chocolate buttercream.
Pipe on the cooled cupcakes.