Vanilla Custard: Ada Boni's Recipe

Vanilla Custard
This is for an Italian style custard perfect for a Trifle, Zuppa Inglese or Baba. It is from one of my favorite cookbooks: Ada Boni Italian Regional Cooking.
Ingredients
5 egg yolks
5 tablespoons sugar
1 teaspoon vanilla extract (Ada called for vanilla powder)
1 rounded teaspoon flour
rind of 1 lemon
2 1/4 cups milk
1 tablespoon butter
Directions
Scald milk by heating up just almost to boiling in microwave, 1 minute/high.

Mix egg yolks, sugar, flour, and vanilla over a double boiler. Add lemon rind and stir in milk gradually beating a wooden spoon.
Cook until it thickens but do not let it boil. It will be smooth and thick but not quite as thick as commercial pudding.
Add butter and stir into the custard.
Remove from heat and discard rind.
Cover with plastic wrap right on top of custard to prevent a skin from forming and allow to cool & thicken up in fridge,