Mix sugar, flour and salt, mix with a fork to break the lumps and set aside.
Beat egg yolks, butter and lemon zest. Continue to beat, add lemon juice. Then, alternating, gradually add the flour mixture and milk, stirring until smooth.
Beat egg whites in a separate bowl. Carefully add them to the dough.
Put the dough in a baking dish and put them on a steam bath. Put in the oven and bake for 45 minutes at 180 degrees. Cool slightly, sprinkle with powdered sugar.