Why did my baked custard curdle?

Custards which are made with milk and whole eggs benefit from resting overnight in the fridge before baking. This is true of flan and crème caramel, which employ hot milk, but especially true for custards that are mixed with cold milk.

Yes, a water bath is always necessary, in custard baking.

Whole egg custard mixtures should always be strained through a fine mesh strainer, to remove any unincorporated parts of the raw egg, especially the chalazae.

The oven temp sounds hot to me, for baking a cold custard base. I don't like my oven to be above 300 for baking custards. Also, every oven is so so different. My 300F oven might be someone else's 275F oven.. I like to play it safe, so I keep an oven thermometer inside. I also like my custard base to be as close to room temp as it can be, before heading into the oven. Sometimes the shock of a cold base + a hot oven will be too much.

Custards are delicate creatures.

I hope this helps. I, too, have seen and tasted my fair share of overcooked custards! I have had to learn a lot of this the hard, expensive way.

Thank you! Over-baking was definitely at least part of the issue.

Hi ktr! A curdled custard is usually a symptom of an overcooked custard. Straining could help make the mixture smoother from the start. And the water bath could help cushion any overcooking. So, both good options! The confectioners' sugar is also catching my eye. Stovetop custards, like pastry cream, often include sugar and cornstarch—the two ingredients in confectioners' sugar—while oven-baked custards usually just include sugar. The cornstarch helps thicken and set the custard, so maybe this recipe is particularly sensitive in the oven. Perhaps try one with just sugar instead?