How to Make It
Brush a 9-by-13-inch baking dish with butter. In a large pot, melt the 5 tablespoons of butter over moderate heat. Add the marshmallows and cook, stirring with a wooden spoon, until completely melted, about 3 minutes. Remove the pot from the heat, add the cereal and stir to coat. Let the mixture stand until cool to the touch, about 2 minutes.
Working quickly, fold in the white chocolate and salt until just combined. Scrape the mixture into the prepared baking dish and press into an even layer. Let stand at room temperature until cool, about 45 minutes. Using a sieve, dust with the matcha powder. Cut into 3-inch squares and serve.