Soak the skinless mung bean overnight, Then rinse under running water and drain thoroughly.
Add the well-drained mung beans and 1.25 cups of water into a nonstick pot. Then bring to a simmer over medium heat. Switch the heat to the lowest and simmer for 20 minutes. If you use skin-on mung beans, the pastry will have a dark green color, and it will take 1.5 hours for the beans to turn soft.
Use a rubber spatula to smash the cooked mung beans until nice and smooth. If you are using the skin on mung beans, you will have to use a blender to crush the beans because the skin is a bit tougher.
Add sugar, honey, salt, and unsalted butter into the mung bean paste. Stir over medium-low heat for 15 minutes to evaporate excess moisture.
When the mung bean paste becomes nonstick, remove it to the side and let it cool to lukewarm. Make sure you cover the mung bean paste so it doesn't form a hard skin.
Divide the mung bean paste into 12 pieces. Roll each piece in between your hands and shape it into a round ball.
Put the mung bean paste into a mooncake presser. Gently press it down onto a clean cutting board with steady pressure. Stay there for a few seconds to establish the shape. Then release it. You will get a pretty, exquisite pastry. Enjoy!