Beat the egg yolks with the sugar and add the grated rind of the lemon and its juice. Pour in the milk and sprinkle with flour on top. Stir the mixture well.
Beat the whites with the salt into snow in a separate bowl. Add them to the egg yolk mixture and stir carefully with a spatula. Don't overdo it.
Distribute the mixture into crème brûlée cups that you've buttered. It's best if they're a bit more shallow so that the cakes bake easier and don't crack on top. But even if they do it's no big deal.
Place them in a tray with a towel on the bottom. Pour in boiling water halfway up the outside of the cups and bake until reddened (about 30 min.) in a preheated 356°F (180 °C) oven.
Serve the cakes cooled and sprinkled with powdered sugar.