I tried making a trifle yesterday, riffing on a recipe from Nigella's Domestic Goddess, and was not happy with how the custard turned out. It was my first time trying to make custard and, given the diversity of recipes, I was wondering if there was a consensus on what was the best method.
Nigella's recipe was
1.25 cups milk
1.25 cups heavy cream (these 2 warmed together)
8 egg yolks
1/4 cup sugar (these 2 beaten together, then the hot milk and cream added to them, and the whole returned to the saucepan and simmered, whisking constantly, until thickened)
vanilla (she said a bean, I used a slosh of Frontier Organics extract)
1/2 cup sherry (boiled with the vanilla until somewhat reduced, then added to the finished custard off the heat)
Other recipes I pulled off the shelf (Cook's Illustrated, Joy, Fannie, Better Homes & Gardens) called for widely varying components--whole eggs, fewer yolks, more sugar, amounts of cornstarch from a little bit up to 1/4 cup.
I followed Nigella's recipe, but the custard never thickened as much as I expected it to--it was soupy, saucelike, and wouldn't have held up as a layer in the trifle. It was also matte-looking, not shiny as I had imagined it would be, and tasted dull. I added a tablespoon of cornstarch and a tablespoon more sugar and kept whisking (cooking it for about 10 more minutes to lose any raw-starch flavor); it thickened dramatically, but got even more oddly matte, and after being chilled it was really a bit too stiff (leading me to wonder if I should have stopped with the apparently-too-thin custard at first), with a slightly grainy consistency, almost like very soft cookie dough.
It still tasted good, though, and I still used it in the trifle, which was, appropriately, a glorious, sweet, alcoholic mess of homemade sponge-cake cubes, very ripe nectarines, chopped crystallized ginger, several splashes more sherry, softly whipped cream, and a homemade caramel sauce drizzled on top. But the custard definitely did not turn out right, and I was wondering if any experienced patissiers out there could scan my (Nigella's) recipe and critique it.
If it was grainy there is a good chance you were cooking it to high a heat and a bit too quickly. Turn the heat down and cook low and slow. Stir with a wooden spoon. When you can draw a track with your finger through the custard on the back of the spoon and it does not fill in, the custard is ready. Chill it well before using.
You should have stopped at the point where NL stopped. You can't judge how thick a custard is going to be until it cools. The best way to do this is to set the bowl in another bowl of crushed ice and continue to stir it until it is evenly chilled. It will thicken quite a bit more in the cooling process. I would not use any cornstarch at all. The egg yolks are the thickening agent and must be cooked thoroughly and slowly.
There's custard sauce (aka creme anglaise) and pastry cream (creme patissiere) which is used as a filling for cream puffs, tarts, trifle etc. The one you tried was custard sauce - it calls for egg yolks, sugar, milk and/or cream and vanilla. Creme patissiere calls for similar ingredients but with the addition of cornflour/cornstarch and/or flour. It's usually cooked over a double boiler, while the creme anglaise is usually not (it can be but it takes longer and it's not really necessary if you're careful). Look for a recipe for pastry cream instead.
The general custard (creme anglaise) recipe I use is
1/2 litre full fat milk (about 4% fat - sorry about the non US measures)
1/2 litre double cream (about 40% fat)
100 gm sugar
6 egg yolks
1 - 2 vanilla pod if required or about 1 tablespoon vanilla extract
Heat milk/cream + vanilla (split pod + scrape out seeds; if using extract add when custard is cooked) + leave to infuse for 30 min - 1 hour.
Heat again, pour onto whipped yolks + sugar + cook, always stirring, until it coats the back of a spoon, or when you blow on it on the back of a spoon, it forms a flower like pattern. If you overcook it to scrambled eggs, you can strain it.
For a trifle, I would add more egg yolks (upto another 6) and increase the proportion of cream to milk), + cook it for as long as possible, to make it thicker. It might look thin when cooked but when cold it will thicken a bit. For a nostalgic taste + colour you could add some yellow custard powder.
Unless you bake it (as in a creme brulee) or add gelatine, custard will not set like the trifles you buy in the shops.