Yoghurt is a western thing, and I've included this recipe, not because it's Thai, but because it contains a common Thai ingredient: coconut milk. You see, yoghurt doesn't need to be made from milk, it can be made from coconut milk too, and if you use the coconut milk powder you can make a much thicker concentrated and creamier yoghurt by using less water than the powder was designed for.
Once you've made it, it's not healthy! Coconut milk is rich at the best of times, and a concentrated coconut yoghurt is best used sparingly as a topping for fruit, or a sauce on a desert!
The main things you'll need, a thermometer that can read 40 degrees Celsius and a live yoghurt as a starter. It must be live, with live bacteria, as the lactic acid making bacteria are the key to a yoghurt. I've used Yolida, a Thai brand. I've also made this using my rice cooker, which I found to be ideal, but a double boiler pan can also be used.