In a medium bowl, mix the rice flour and sugar until combined, then stir in the water until smooth.

Cover the bowl with a plate or plastic wrap and microwave for 1 minute. Stir the mixture, then microwave it for 1 minute. Stir the mochi mixture once again and microwave it for 30 seconds, until thickened and very sticky. Set the mochi in the fridge for 30 minutes to cool.

Peel and slice the mangoes into 8 cubes around 1 inch wide. Heavily dust a piece of parchment paper with cornstarch and divide the cooled mochi into 8 pieces.

Dust a rolling pin with cornstarch. One at a time, roll out the mochi pieces into a circle large enough to enclose a cube of mango.

Place the mango in the center of the rolled out mochi, then wrap the mochi up and over the mango, pinching it to seal. Repeat with the rest of the mangoes, dust the mochi with cornstarch if needed, then serve the day of (refrigerating mochi will dry them out).