First, wash and cook the white potatoes. You can cook them in a steamer for 30-40 minutes, or in the microwave (see microwave method here) for 10 minutes, or until a toothpick can be easily inserted into the center. Mash using a fork while the potato is still warm.
Preheat oven to 350 F. Grease a 8 x 8 inch square cake (I used a Pyrex glass dish) with some oil (spray oil works fine, too).
In a clean bowl, mix glutinous rice flour (mochiko flour) with tapioca flour, sugar and salt.
Knead the cooked mashed potato into the dry ingredients best you can, so that the potato is well incorporated with the flour. Set aside.
Pour the hot water** into the flour mixture and use a whisk to mix well.
Stir the coconut oil into the batter
Stir in the add ins (green onion and diced potatoes)
Pour batter into the greased cake pan or oven safe pan, and bake in oven 350F for 50-60 minutes , or until the middle is set and the edges are crispy.
For softer texture, you can also steam the mochi batter on medium heat for 45-60 mins, or until the center of the mochi cake is set.
Let cool completely before serving, or cool the cakes in the freezer for 2-3 hours which helps to get clean slices of cakes.
Enjoy!