Preheat the oven to 160°C fan / 180°C conventional. Line a 12-hole cupcake tin with 12 cases.
In a large bowl, beat the butter, sugar and vanilla together until light and fluffy.
Beat in the eggs one at a time, adding 1 tbsp of the 150g flour with each egg to stop the mixture from curdling.
In a separate bowl, stir the remaining flour and cocoa powder together to combine. Then tip into the butter mixture and fold until combined.
Fold in the milk and boiling water to make a smooth glossy chocolate mixture.
Divide evenly between the cupcake cases, filling each one ¾ full.
Bake for 20-22 minutes until the cupcakes are risen and spring back when lightly pressed in the middle. Transfer to a wire rack and leave to cool completely.