Custard Fillings (without custard powder) [奶黃餡]

Last week, I tried to make Chinese steamed bunny buns with custard fillings. I never made custard fillings. So, I browsed the internet to look for a custard fillings recipe. However, all the recipes I found have to include custard powder. I went out and bought a can of custard powder only to find out this can of powder just contains some corn starch, artificial color, artificial flavor and a few unrecognized chemical names but not a trace of egg in there.  Fortunately, I found a very good recipe from one of my cookbook collection, Mastering the Art of French Cooking, Vol. 1 by Julia Child. This is really a back to basic recipe which uses all natural ingredient and it tastes really good. Of course, it takes more work to prepare than using custard powder, but I think it's worth it.

Ingredients: (yield 230g of custard fillings. enough to make 12 small steamed buns)
2 Eggs
50g Sugar
25g All purpose flour
100ml Evaporated milk
100ml Milk
1/2 Tbsp Vanilla Extract or a string of vanilla bean
1/2 Tbsp butter

1. Beat the eggs with an electric beater or wire whip in a mixing bowl, and gradually add the sugar. The mixture will eventually turn to pale yellow. It will also be thick enough that when you lift up the beater, the mixture will fall back into the bowl forming a slowly dissolving ribbon on the surface of it. This process can prevent the egg yolks from becoming granular when heated.
2. Beat in the flour gradually. 
3. Boil the milk + evaporated milk (if you use vanilla beans, add to the milk mixture now)
4. While still beating the mixture, gradually pour in the strained (if using vanilla bean) boiling milk in a thin stream of droplets.
5. Pour the mixture to a saucepan. Set to moderate high heat. Constantly stir the mixture with a wire whip. Remember to reach all the way to the bottom of the pan to make sure it's stirred thoroughly.  As the sauce comes to the boil, it will be lumpy. Turn to low heat and continue to stir until the flour is cooked, about a minute.
6. Remove from heat immediately and stir in the butter. **If you use vanilla extract, add this now.

Note:

1. The original recipe calls for milk only, but since Chinese likes the smoothy texture and strong taste of evaporated milk, I substituted the evaporated milk to half of its recipe. I used 2% fat milk. If you prefer to have a stronger taste of milk, use whole fat milk.
2. My average-Joe palates can't tell the difference between vanilla beans and vanilla extract. The reason I used the beans in this recipe was I wanted to keep the color of the custard look more bright yellowish. I tried to use extract in my last trial, but it tainted the custard and it looked kind of on the brownish yellow side. It indeed was a minor issue, but I wanted to look good on a photo shot, so, I used the expensive beans (yes, beans are way more expensive) this time :P
3. I usually use less sugar in my recipe and this one is not an exception.
4. Thin stream of droplets means pouring it very slowly. I tried to take a picture of it but it's very hard to get a clear picture while I need to perform both tasks at the same time.
5. It is easy to overcook the mixture. You may have to try making it several times before you can get the right consistency, not too thick but not too thin either :P don't give up!