I finally found these mochi puff premix from a local Korean grocery store. There is not English baking instructions but luckily there are pictures for guidance too. The instructions are pretty simple, just add water and one egg to the premix, knead and bake. These treats are super yummy once they came out from the oven, while center is soft and slightly chewy and crispy on the crust. As they cool down, they gets chewier and chewier and the crust eventually gets soft.
Anyhow, this is what I have been trying to replicate after all my mochi puff trials. I’m not giving up yet, wish me luck!