Custard is the controlled coagulation of egg proteins in a liquid (milk, cream, or both). Stirred custards (crème anglaise, pastry cream) are cooked on the stovetop while stirring constantly. Baked custards (crème brûlée, crème caramel, quiche) set in the oven in a water bath. The difference between silky custard and scrambled eggs is temperature control — egg yolks set between 65-80°C. Every degree matters.
For crème anglaise (stirred): heat milk with vanilla, temper into egg yolks and sugar, return to low heat, stir constantly with a spatula until it coats the back of a spoon (82-85°C). Never boil. Strain immediately. For pastry cream (crème pâtissière): same start but flour or cornstarch is added — this raises the boiling point of the egg proteins, so it MUST be brought to a full boil to activate the starch and deactivate an enzyme (amylase) that would thin it out. For baked custards: the water bath (bain-marie) moderates oven heat, ensuring the custard never exceeds 85°C internally.
Tempering: pour a thin stream of hot milk into the egg mixture while whisking constantly — this gradually raises the temperature without shocking the proteins. For crème brûlée: bake at 150°C in a water bath until the custard jiggles like set jelly when tapped — firm edges with a slight wobble in the centre. Chill completely (4+ hours) before torching. The sugar layer should be paper-thin and crackle when tapped with a spoon. For pastry cream: press plastic wrap directly onto the surface to prevent a skin.
Boiling crème anglaise — eggs scramble. NOT boiling pastry cream — the starch doesn't set and the amylase thins it. No water bath for baked custard — the edges overcook while the centre is raw. Not tempering eggs before adding hot liquid — you get sweet scrambled eggs. Not straining — any egg bits ruin the texture.