Preheat oven to 350 degrees F and grease an 8x8" baking dish.
Mix all ingredients in a large mixing bowl, stirring well to ensure no dry lumps of flour remain.
Pour into your prepared baking dish and bake for 60 minutes or until a toothpick inserted in the center comes out clean.
Allow butter mochi to cool before slicing and serving. Leftover butter mochi can be kept in an airtight container in the fridge for a few days or kept in the freezer for a few months.
You can double this recipe and bake it in a 9x13" baking dish for one hour.
I highly recommend using a digital kitchen scale to measure your mochiko. Differences in the amount of mochiko may result in differences in the thickness of your batter, which will affect bake time.
Feel free to substitute ogura an (chunky red bean paste) for the koshian if you'd like to have pieces of red bean throughout your butter mochi, as opposed to a smooth texture.