In a bowl, mix eggs and sugar until it has turned pale yellow and slightly stiff.
In another small bowl, add the zest and juice of 1 small lemon (should weigh about 30-35g), then top up with milk until it weighs 50g total. Add this lemon + milk mix to the eggs & sugar bowl.
Give everything a good mix, then add in the flour & baking powder. Mix with a spatula until well incorporated, then add in melted butter. Mix again!
Cover the bowl of batter with cling wrap, then stick it in the fridge for 24hrs.
Next Day - 30 min before baking, take batter out of fridge. Let it come back to room temp.
Preheat oven at 220*C. Butter the Madeleine Moulds generously, then pipe the batter onto the mould. Let it cook in the oven for 8-10min until the top of the Madeleine has peaked and set, and the bottom is lightly golden.
