Drain the water and put the cashew nuts in a blender along with the rest of the ingredients. You can add or omit orange juice as needed in order to achieve your desired consistency.
Story Behind this Recipe
This is based on a recipe I found on a blog, but I altered the ingredients to ones that are commonly sold in Japan.
Old and less powerful blenders might not be strong enough to blend the nuts completely, and you'll be left with lumps in your frosting, so unless you prefer the texture, make sure your blender is powerful enough to create a smooth cream.