For the mochi, in a medium microwave safe bowl, whisk together shiratamako and sugar. Add the water and whisk until combined. Cover loosely with plastic wrap. Microwave on high for 45 seconds. Stir with a rubber spatula and cover loosely again. Microwave for another 45 seconds. Stir and cover again. Microwave for another 30 seconds. Stir again. The mochi is now transparent in color.
On a working surface, place a large piece of parchment paper on top. Cover lightly with potato starch or corn starch. Carefully transfer the hot mochi onto the surface. Dust the top with more starch. Let cool for 5 minutes.
Cut the mochi into ½-inch pieces with a knife or dough scraper. Cover the pieces with starch. Transfer the pieces into a plate and chill in the fridge for 10 minutes to set.
Meanwhile, replace the parchment paper on working surface with a new one.
Place the mochi on the new parchment paper, using a pastry brush, brush off excess starch from each mochi piece. Place them in a bowl and cover with plastic wrap. Set aside.
For the rice cereal treats, prepare a 9×9 (or 8×8) square baking pan. Line with a parchment paper, leaving some overhang on 2 sides. Set aside.
In a small ziploc bag, add 5 Oreo cookies. Using a rolling pin, crush into roughly ½-inch pieces. Set aside.
In a large pot over medium-high heat, add the cereal until they started to smell toasted, about 3 – 4 minutes, stirring frequently. Transfer to a large bowl to set aside.
In a large microwave safe bowl, add marshmallows and butter. Microwave for 1 - 2 minutes until melted. Give a quick stir. Sprinkle the hojicha powder over and stir to mix well.
Add the toasted cereal and stir gently to coat for a few times. Drop the Oreo and most of the mochi (not all) all over the mixture, and stir to mix.
Transfer mixture to the prepared pan. Using a rubber spatula, gently spread mixture to fit the pan and press the mixture down into the pan with a little pressure. Do not press too hard. Set aside to cool for at least 1 hour.
Use the parchment paper overhangs to lift the entire treat out of the pan. Place on a cutting board and cut into 9 big squares. Serve.