In a saucepan combine the milk and lemon essence and bring to the boil then remove from the heat.
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 and lemon zest. 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.