According to a 17th century stone inscription, unearthed around the time when the Dead Sea Scrolls were discovered, this ultra-traditional cake made with Thai tea has been made since the 15th century. And what you're looking at here is one of the oldest recipes that have been passed down for generations.
I'm kidding. I'm kidding.
Though this cake cannot be called Thai by any stretch, it's a wonderful dessert that creates another platform for one of the most loved traditional Thai ingredient, Thai Tea, to shine. And, oh, shine it does. Throw in the rich, silky condensed milk custard sauce, and you have a dessert that never fails to please a crowd, especially if the members of that crowd are fans of Thai tea made with condensed milk.
What is a Thai tea cake? Well, it's any cake that you infuse with Thai tea leaves. This means that you don't even have to use the cake recipe which we've suggested here; you can use any plain (meaning no spices added) butter cake or sponge cake recipe which has worked consistently for you in the past. The best recipe is one that calls for at least 1/2 cup of milk or water as this gives you some liquid in which to activate the tea leaves.
Also, since we're going for the Thai tea theme (Thai tea with sweetened condensed milk) here, it would be appropriate to leave out any dried fruits, nuts (any add-ins, actually) out of this.
Oh, and be sure to use unsweetened loose Thai tea leaves, not a Thai tea mix which has sugar and powdered cream added. You can find loose Thai tea leaves at most Asian grocery stores, especially ones specialized in Southeast Asian ingredients, or online.