This is great if you found a great deal on lemons. When life gives you lemons, make lemon pudding.
Whisk the sugar and cornstarch together in a medium saucepan. Whisk in the milk, egg yolks, lemon zest and salt. Stir regularly over medium heat, then constantly near the end, until the pudding thickens enough to coat the back of a spoon. The goal is to get it thick, but not boiling. If you go to boiling you might curdle the eggs, which will still TASTE OK, but will have a weird texture.
Take the mixture off the heat. Whisk in the butter, then whisk in the lemon juice. If you are serving the Queen, strain through a wire sieve, but if it's just for you and your family, pour into a bowl to cool, or individual glasses (if real glass, warm them with hot water first, then dump out, so that you don't crack the glass).