Preheat oven to 350 degrees.
Grease and flour a cake pan (square, round, 8-9 inch is fine).
In a saucepan, mix the sugar with the milk over low heat. Add the vanilla extract and make sure the liquid is only simmering and not boiling. Keep mixing every 5 minutes or so and let simmer for 40 minutes. Set aside to cool completely.
Beat the egg yolks together until creamy and add the flour, making sure to incorporate into the mixture completely.
Very slowly, add in the cooled milk and whisk vigorously as you add the milk. The mix might try to separate, but keep whisking and it will come together, as long as you add the milk slowly. Set aside.
Whip the egg whites until soft peaks form and, by hand, fold the egg whites into the egg yolk/milk mixture and keep folding until the whites are fully mixed in, without letting the mix go flat.
Pour the batter into the prepared pan and bake for 30 minutes or until the outside is golden brown, or where you want the consistency. You can check with a toothpick for a little custard or a clean toothpick, and take the cake out when it is to your liking. Allow to cool in pan and dust with confectioners sugar and/or orange slices for garnish.