Beat the butter with half the amount of sugar and the lemon zest until creamy. Add egg yolk, flour, baking powder, lemon juice and milk and keep beating.
Beat the egg whites until stiff, add the remaining sugar and keep beating until the mixture is glossy.
Fold the egg white mixture into the egg yolk mixture and pour into an ovenproof bowl. Place the bowl in a double boiler in the oven and bake for about 50 minutes, until it is light golden brown.
The clue is that a soufflé-like sponge cake layer stands out when baking. A delicious, light lemon sauce collects underneath.