In a saucepan combine the milk and vanilla bean and bring to the boil then remove from the heat. Remove the bean, split it in half and scrape out the seeds. Add the seeds back to the milk. Place the pod back in the milk and let infuse 20 minutes then discard the pod.
In a bowl, beat together the egg, sugar and custard powder. Add to the milk, stir and return to the heat, whisking until it thickens. Let cool. Place plastic wrap on the surface of the custard so no skin forms.
When cool, whisk in the creamed butter. Transfer to a piping bag and use to fill macarons. Let rest in the fridge 1 day before eating to allow the flavours to infuse.