Divide the rice flour into 2 equal amounts. Add the matcha powder to only one of them so you get green and white rice flours.
Warm up and soften the butter in a microwave for about 20 seconds.
Add the egg yolk and sugar with the butter and mix well.
Separate the buttercream into 2 equal amounts.
Add the green matcha rice flour with one of the buttercream to make green cookie dough.
Add the white rice flour with another buttercream to make white cookie dough.
Put the cookie dough in a refrigerator and leave them for 30 minutes.
Take the dough out of the fridge and shape them into logs to make a spiral pattern.
Wrap the dough with a plastic wrap and put in a freezer for about 30minutes until it has set.
Take the cookie dough out of the freezer and slice into pieces.
Bake them in a preheated oven at 170℃ or 340 °F for about 20 minutes.
Cool down the cookies at room temperature and serve.