If you whisk eggs into a quantity of cream and heat it gently, the proteins in the eggs will eventually begin to firm up just as they do in a skillet. Because the eggs are dispersed throughout the cream, they'll produce a soft, thick gel rather than the familiar fried-egg texture. Liquids thickened this way are called custards, and they make delicious fillings for baked goods. If they're baked, however, the eggs contract and lose their ability to thicken the mixture. pastry cream addresses this by thickening the cream with flour or cornstarch as well as the eggs, making a stable custard that can be baked or used cold.

If you make up a batch of pastry cream and store it in a covered container, you might find after a day or two that it's developed a hard, leathery layer at the surface. If you've left any air space above the surface of the custard, this is normal. Moisture evaporates from the surface, drawing the custard's proteins closer together and forming the distinctive "skin." This can be avoided by pressing plastic wrap directly to the surface of the pastry cream as soon as it's cool enough to refrigerate. If a skin has formed on your pastry cream, lift it off carefully and use the remaining custard as usual.

Occasionally, you might find that your pastry cream has set into a single, solid lump in your storage container. This is almost always the result of an error in the written recipe, or not measuring your thickening starch accurately. Pastry cream recipes typically use flour, cornstarch or both. These starch-based thickeners continue to gel the mixture as it cools, and if you've used too much your cream will become unusably stiff. Most recipes use 1 1/2 to 2 tablespoons of cornstarch for each 1 to 1 1/2 cups of cream. If your recipe calls for significantly more, it's probably an error. Try scaling back the amount of starch in future batches, or use a different recipe.

If your pastry cream is thick and pasty but still spreadable, it's just slightly past the normal proportions of starch. You can usually make it usable by whipping it for a few seconds in a stand mixer or food processor, which will soften and aerate it enough to use. You can lighten it further by drizzling in a small amount of heated cream. For pastry cream that's set to a solid consistency, heat 1/4 cup or 1/2 cup of cream in a small saucepan and whisk in the thickened custard a spoonful at a time after each addition has dissolved completely into the mixture. Continue until the full amount is usable, adding extra cream if necessary. Strain out any lumps, and use the custard as usual.