I'm pretty good at desserts, but custards still vex me.

Maybe there's a trick I'm not getting, or my definition of, say, boil,
is different than what the cookbook is telling me.

For instance, yesterday I made a dacquoise. For the most part, it came
out very well, but I think I can do better on the custard part.

Here's the pertinent part of the recipe:
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1. bring 2 cups half-and-half just to a boil in a large saucepan over
medium heat, then turn off the heat, reserve.

2. in a mixer, whip 8 yolks, 1 tsp vanilla and 1/2 cup granulated
sugar for about 4 minutes. when you lift the mixer blade, the mixture
should form a thick ribbon. slowly mix in 1/2 cup all purpose flour
until well combined.

(me - I've learned that when the mixer is done to get a spatula and
ensure that all of the flower has been well incorporated. Sometimes
the beater forces some flour to the bottom or side and makes a kind of
sludge.)

3. add 1 cup of the hot half-and-half to the bowl and mix well.

4. turn the heat to medium again under the pan of half-and half.
Showly, mixing constantly, pour the yolk mixture into the saucepan.
Bring just to a boil, stirring the entire time. Reduce the heat to low
and simmer for about 3 minutes, stirring constantly. Allow to cool.
Pour into abowl, cover with plastic wrap and refrigerate until ready
to use.

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Maybe the book's concept of boil on point 4 is different than mine,
but to me, it seems to congeal long before it comes to what I consider
a boil. I've tried whipping it with a whisk rather than use a spoon,
but I find too much sticks to the corners or the pan and begins to
become something close to scrambled eggs.

Should the H&H mixture be closer to a boil before I start pouring in
the yolk mixture? (generally I boil the H&H first, take it off the
heat and cover it, then prepare the yolks and temper the yolks with
the H&H and THEN turn on the stove again. So, usually, 15 or so
minutes have passed since the H&H was on the flame.)

Anyone else have any secrets of the custard trade?

For the record, my custards almost always taste good, and rarely
curdle, but it's usually a bit lumpy and I know I can do better.

Many thanks!

PS - btw, apart from cooking the egg yolks, why does one want to boil
milk/cream/H&H in custards and other simialr desserts? What chemical
process occurs that aids in the cooking that you wouldn't have if you
just used cold dairy?