Generously grease a 9x13-inch microwave-safe pan with cooking spray.
Combine mochiko and sugar in a bowl. Add water, coconut milk, and vanilla; stir everything together until smooth. Pour batter into the prepared pan.
Microwave on high for 18 minutes until cake is set. If the top center of the cake is still sticky, microwave in increments of 2 minutes until no longer sticky.
Remove the pan from the microwave and set on a cooling rack. Cool for about 90 minutes.
Cut cake into 16 pieces and roll in potato starch or kinako.