In a small, microwave safe bowl, whisk together flour, cornstratch, sugar and milk until smooth. Tightly cover with plastic wrap, leaving a small opening on one side of the bowl to allow steam to escape. Microwave for 30 seconds.
Remove from the microwave and use the spatula to stir in between. Return to the microwave for another 30 seconds and repeat once more, for a total of 3 rounds in the microwave. The mixture should be mochi-like and pull away from the sides of the bowls. If not, return to the microwave and repeat once more.
Finally, add vegan butter to the mixture and use the spatula to incorporate into the mochi. It may take a little bit of effort but will eventually work its way into the mochi.
Transfer the mochi to a nonstick baking mat or nonstick surface and divide the mochi into 12 pieces. I found that working with about a tablespoon and a half of mochi works best and you may end up with a little bit of mochi leftover.