How can I prevent the custard from curdling or scrambling while it cooks?
Temper the yolks first by whisking in a small amount of the hot milk, then gradually add the rest so the eggs warm slowly instead of shocking and coagulating. Cook the mixture over low to medium-low heat, stirring constantly, and pull it off the stove as soon as it reaches about 175 °F / 79 °C; immediate straining into a bowl set over an ice bath stops the cooking and keeps the custard perfectly smooth.
How far in advance can I prepare the custard, and what’s the best way to store it?
Prepare the custard up to 3 days ahead; once strained, press plastic wrap directly onto the surface, seal the bowl or transfer to an airtight container, and refrigerate at ≤ 40 °F (4 °C). For longer storage, freeze it flat in a zip-top bag for up to 1 month, then thaw overnight in the fridge and whisk smooth before using.
How can I fix a custard that’s turned out too thin or too thick?
If the finished custard is too thin, return it to gentle heat and cook a little longer, stirring until it reaches 175 °F/79 °C; if needed, whisk 1 tsp cornstarch into 2 Tbsp cold milk, add it, and simmer for 30 seconds to thicken. If it’s too thick or has set up after chilling, briskly whisk in a splash or two of warm milk or cream until the texture loosens to the desired pourability.