Grease an oven dish with a little butter. In a slightly warmed baking bowl, knead together the remaining butter and flour until well combined.
In a medium saucepan, bring milk and sugar to a boil. Stirring vigorously (to prevent lumps from forming), add the flour and butter mixture and stir on the stovetop until the mixture pulls away from the skillet. Pour the batter into the baking dish form and cool slightly.
Wash the lemon with warm water and dry it. Finely grate the peel and squeeze the juice. Divide the eggs exactly so that the yolks remain whole.
Stir one egg yolk at a time into the batter. Pour the juice of one lemon through a sieve and add the grated zest. Mix well.
Whip the egg whites to egg whites and add the vanilla sugar towards the end. Fold loosely into the dough mixture. Pour into the oven dish and place in the bottom half in the oven at 180 °C. Bake for thirty-five to forty-five minutes.
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