Baking isn't exactly rocket science, but it does require the willingness to master a handful of basic skills. Making custards is one of those skills. They're not just fine desserts in their own right – crème brûlée, anyone? – but also make a vital part of other recipes, from ice cream and cream puffs to your granny's coconut cream pie. Custards are mostly thickened by eggs, which is what makes them custards, but other thickeners are sometimes used.
You Can't Hurry Custard
It's important to understand that you can't make homemade custard in a rush, at least not until you've done it often enough to take calculated risks. If you try to speed things by using high heat, you'll either scorch your cream or cook your eggs so they separate from the custard in unsightly lumps.
That's why most recipes call for a careful process of "tempering" the eggs, and the crucial thickening stage usually takes place afterwards over a gentle water bath or double boiler. First, you mix the eggs with the sugar the recipe calls for, then you gently heat them by pouring the hot cream into the bowl in a thin, slow stream. The cold eggs dilute the hot cream, which raises their temperature slowly. After you incorporate 1/4 to 1/3 of the cream, you can stir the egg mixture back into the hot cream in the same slow, steady stream.
Once it's all combined, keep stirring the custard over a pan of hot water until it thickens fully. A flexible silicone spatula works best because the eggs will begin to cling to the bowl as they get hotter. If you leave them there, they'll cook and make a thin layer of sweet omelet on the bowl, which is not at all what you want. To get it right can require several minutes of patient stirring, so hang in there. Often, if your custard isn't thickening, it's because you simply haven't cooked it long enough.
Adding Eggs for Thicker Custard
If you've cooked your custard for as long as it's supposed to cook and it's still not as thick as you'd like, you may need to up the egg content. Adding a whole egg will give a firmer texture, while adding yolks gives you a thick but soft texture and extra richness.
If you're rescuing a custard that's cooking right this minute, whisk your extra egg or yolks in a separate bowl and repeat the tempering process, pouring in the hot custard mixture in a very thin, fine stream. Once they're combined, return your newly enriched custard to its double boiler and cook until it thickens.
If you're starting a recipe that you've made before, and found that it wasn't as thick as you like, add the extra egg to the sugar at the very beginning and proceed normally. If you wish, you can reduce the cream by 1/4 cup to compensate for that extra egg.
Pastry Cream or Crème Pâtissière
Custards by definition get some or all of their thick texture from eggs, but that doesn't mean you can't use other ingredients. Thick custard recipes often add thickening power in the form of a starch. Usually that will be cornstarch, because it thickens quickly, but your recipe may call for flour, arrowroot or some other starch instead. It's usually stirred into the sugar, which helps it disperse evenly in the custard without forming lumps as it heats.
Adding starch to the custard has a couple of crucial benefits. One is that it makes the custard a lot less finicky to work with. The starch stabilizes the eggs, so they're not as prone to cooking out and causing the custard to "break." The other is that the finished custard is thick enough to serve as a sliceable filling in cakes and pastries, which is why this kind of custard is usually referred to by the French term crème pâtissière or its English equivalent, pastry cream.
Gelatin As a Thickener
Some custards and custard-like dishes use plain, unflavored gelatin as a thickener. Many classic European desserts call for a type of custard called Bavarian, or Bavaroise, which is made the traditional way with eggs and milk or cream, but has gelatin added later in the process to give it a firmer texture that's suitable for slicing.
The popular Italian dessert known as panna cotta uses no eggs at all. It's thickened entirely with gelatin, which – when it's done right – gives a silky, custard-like texture but with a lighter, cleaner flavor.