Drain the cashews and add them with the water to a high-speed blender. Blend on high for 30-50 seconds or until smooth. Transfer to a bowl and set aside (do not refrigerate it).
Matcha Filling
Melt the cacao butter in a small saucepan. Remove from heat as soon as it is melted. The cacao butter should not be hot. If it's too hot, let it cool for a few minutes or until room temperature.
Transfer the melted cacao butter to a small bowl. Add the powdered sugar, matcha, cashew cream, and vanilla. Whisk until fully combined, and keep whisking for 2-3 minutes or until it has slightly thickened (like a condensed milk consistency).
Transfer to the refrigerator and refrigerate for about 2 hours or until set.
Mochi Dough
In a small bowl, whisk together the sweet rice flour, white sugar, almond milk, oil, and matcha powder.
Bring a pot of water to a boil and place a bamboo steamer basket on top. Transfer the mixture to a container that will fit inside your bamboo steamer.
Close with the lid and steam for 20 minutes. Halfway through steaming, stir using a spoon (you will see some liquid and pieces of dough, that is normal, just give it a good stir).
After 20 minutes, let the dough cool for 15-20 minutes or until just warm. Transfer to a bowl and stir well using a wooden spoon until the dough is smooth.
Wrap the sticky dough in plastic wrap and refrigerate for about 45 minutes.
To shape: Scoop out 1.5 teaspoon balls of the matcha filling, and carefully roll them into balls, set aside. Dust a working surface with cornstarch. Take a small amount of mochi dough (about 2 tablespoons) and flatten it into a round on the dusted surface.
Place a ball of the filling on the center of the dough and wrap the dough around the filling. Pinch the edges to seal. Transfer to a plate with the sealed face facing down. Repeat with the remaining filling and dough.
Enjoy immediately, or refrigerate for a couple of hours. Mochi is best served the same day but will keep for up to 3 days in the refrigerator.
Notes
For the filling: if you have access to vegan white chocolate: Finely chop 100g of white chocolate and transfer to a heatproof bowl. Add 3 tbsp of cashew cream. Heat in a microwave, stirring every 30 seconds, until the chocolate is melted and smooth. Stir in the matcha powder and refrigerate for at least 2 hours or until firmer.
To make the dough in a microwave: Whisk together the sweet rice flour, white sugar, almond milk, oil, and matcha powder in a microwave-safe bowl. Cover the bowl with plastic wrap and place in the microwave for 1 minute on high heat. Stir the mixture using a rubber spatula and microwave again for 1 minute or until it forms a soft and sticky dough.