That's a lot of corn starch in the pastry cream. I made it anyway, but it really doesn't taste good. Will it improve as it cools? Should I star...

Most importantly, Emiko should be informed so she can change her recipe!! I'll email her.
BTW, many recipe chefs rarely see the hotline or check the comments on their recipe pages. So when i have a question that i really need ansswered, i click on the chef's recipe and then from there i click on their name; and then on their member page, i send them an email by clicking on the envelope symbol. those emails go to them directly.

Cornstarch in pastry cream is quite common, which is why it is classified as a "cream," not a custard (a custard being that which is set solely by the coagulation of egg protein). That said, the formula I use calls for 2.5 ounces of cornstarch, 15 egg yolks, 8 ounces of sugar, and 32 ounces of milk. The recipe you used calls for a little more cornstarch (about 2.75 ounces), but nearly half as many yolks and less sugar for the same amount of milk - 32 ounces. Increasing the quantity of yolks will certainly help with the consistency, and more sugar will boost the flavor. It is important in cooking pastry cream to be certain to bring it to a boil, then maintain it for a minute, stirring all the while with a flat-edged rubber spatula (it's stiffer than silicone) that maintains continuous contact with the bottom of the pot so as not to scorch the contents. The boil will cook out the starchy taste of the cornstarch.

The recipe you used for Zuppa inglese is off. Use Julia's recipe!

The 3/4 cup of cornstatch must be a typo. You don't need any cornstarch at all, really. the milk and egg yolks are enough to thicken the custard. At most you might use a few teaspoons, never 3/4 cup!

I haven't looked closely at the recipe you used, but I have never used cornstarch in pastry cream. I don't know why you would.