Melt the white chocolate in a bain marie. It shouldn't get warmer than necessary. Stir in the matcha powder until it has the desired shade of green.
Dollop a little bit of the melted chocolate with a small spoon into each of 9 flowers in a silicone mould. Then add the rice flakes. Then carefully pour the remaining matcha chocolate on top. The filling should end with the mould and have clean outlines.
Knock the mould on the table a few times so trapped air bubbles can come up, then put the mould in the freezer for 15 minutes.