How to make a smooth and creathe apple custard

Custard should taste eggy. This is a feature, not a bug. However, from your mention of flour, I think what you're talking about is a souffle, not something like a creme brulee or along those lines. Would that be correct?

Edited based on subsequent clarifications:

Ah. Your method is sorely lacking, and your ingredients are off. You need to temper the yolks first, you need to use yolks only, and use cream instead of milk. Here is a smaller version of what I use at work for creme brulee: - 1L 35% cream - 11 egg yolks - 1C sugar - 1 vanilla bean, scraped

Bring cream to a simmer with the vanilla seeds and pod (you can use approx 1.5tsp vanilla extract, pure not artificial, instead). While it is heating, beat the yolks and sugar together until just incorporated. Pour the cream into the eggs whisking briskly to prevent curdling. Pour mixture through a fine chinois. Skim off foam, pour into ramekins. Place ramekins in a pan, add water to 1/2way up the sides. Cover pan with foil. Bake at 275 (convection) or 325 (non convection) for approx 40 minutes, until wobbly in the centre but set. Chill until set, eat.

The deflation is caused by a souffle effect coming from including the egg whites, which are never used in a custard--custards, creme anglaise, etc, are always yolks only. When you include the whites, air will be trapped inside temporarily, and will escape/collapse when the mixture cools. If you bake until fully stiff with my method, you will get hard and rubbery custard by the time it is set in the fridge.

Also, save the whites--they freeze well--for meringues or souffles.

Most creme brulees require baking, however after a little research I did find a recipe in "On Cooking" (Sarah Labensky/Michael Hause) that came from Chef Vincent Guerithault of Vincent on Camelback in Phoenix, AZ and his was similar in that it was not baked.

First, just making creme anglaise with heavy cream isn't going to do anything to let it set up into a firm custard. More egg yolks or starch would be needed.

Supposing that this really does work and it was something you perhaps did, my guess would be that it was either mixed too much (breaking down the proteins trying to link together) or too vigorously (incorporating air which weakened the protein links). In your description you say you "beat it". Did you beat it or stir it? It should be stirred back and forth zig-zagging across the bottom of the pan with a wooden spoon or heat-safe rubber spatula to keep from whipping air into it.

Time, temperature, and eggs/dairy ratio are going to be the main issues in getting custards to set.

Egg proteins begin to set at 160 degrees but curdle at 180 so there's very little "wiggle" room temperature wise.

According to Shirley Corriher's "Cookwise": 2 egg yolks will just barely thicken 1 cup of milk or cream. Her Creme Anglaise recipe uses 5 egg yolks to 1 cup of milk and 1/2 cup heavy cream which is more yolks and less liquid than Julia's and this isn't intended to set up. 1 teaspoon of starch isn't going to provide the thickening power that is needed, it's there to keep the yolks from curdling as easily.

The recipe I use and many others I've referenced (including Chef Vincent's), use a ratio of about 6-7 yolks per cup of cream.

Also, if using a starch, you need to nearly bring the custard mixture to a boil (as is common in puddings and cream pie fillings) otherwise an enzyme in egg yolks known as alpha-amylase will eat away at the starch bonds and break them down into a watery mess. Chef Vincent's does not use any starch.

If you want to use that recipe, I would increase it to 10 egg yolks. After the hot cream is tempered into the egg yolks then return to the heat and cook, stirring constantly, until very thick but do not let it boil. Remove it from the heat and strain into a clean metal bowl and chill over an ice bath to cool quickly. Once cool, spoon into your desired serving dish or a cookie cup and caramelize the top with sugar.