What are the core ingredients of crème pâtissière and how is it thickened?

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Crème pâtissière thickens through a starch-thickened custard base made from milk, egg yolks, sugar, and a starch. The starch (usually cornstarch or a small amount of flour) swells and gelatinizes when heated with the milk, creating a smooth, cohesive texture that coats the spoon. The eggs add richness and help set the structure, but the primary thickening action comes from the starch as it heats. Tempering the yolks prevents curdling, then cooking the mixture until it thickens to a nappe ensures the starch is fully hydrated and the custard holds its shape as it cools. If you only had milk and sugar, there’d be no thickener to develop that firm, silky body; using water isn’t the typical base, and thickening by air or by protein isn’t how pastry cream is made.