Spray an 8-by-8-inch baking dish with nonstick cooking spray.
In a medium saucepan, melt the butter. Add the marshmallows and stir until they melt. Remove the pan from the heat, add the puffed rice cereal and matcha powder and mix to combine. Press the mixture into the prepared dish and let stand until the treats have set, about 10 minutes. Cut into 2-by-2-inch squares and transfer to a plate.
Set a wire rack on a baking sheet. Melt the dark chocolate and coconut oil together in a microwave or double boiler. Dunk the bottom of each square into the melted chocolate mixture to coat and place on the wire rack to set.