Wash, peel, and steam the taro and purple sweet potatoes for 30-40 minutes until easily pierced with a toothpick; then, while still warm, mash it with a potato masher or fork until smooth and chunk-free.
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 and sugar.
Knead the cooked taro and purple sweet potato into the dry ingredients best you can, so that they are well incorporated with the flour. Set aside.
Heat almond milk in a microwave safe bowl or glass measuring cup until hot, about 2 minutes. (This is an optional step to make the mochi chewier. Read notes***)
Pour the hot almond milk into the flour mixture and use a whisk to mix well.
Stir the coconut oil into the batter
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!