In a large stockpot or very large skillet with deep sides, melt butter and add salt.
When butter is melted, add marshmallows and condensed milk. Keep heat on low-medium and stir until marshmallows have melted. Don't let it bubble!
When marshmallows are mostly melted, sift matcha on top. Stir until the green color is evenly dispersed.
Turn off the heat and add the cereal. Stir gently to combine.
Pour Rice Krispies into a greased and lined 9x13" pan. Flatten and press down gently with a spatula. For very thick Rice Krispies you can use an 8x8" pan instead.
Cover and let cool down at room temperature for at least 30 minutes before slicing into squares or rectangles for serving. Leftovers keep in an airtight container at room temperature for several days.