Generously grease a 13-by-9-inch baking pan with butter, or butter and line with parchment paper for easier removal of the treats later.
Place rice crispies cereal in the largest mixing bowl you have.
In a medium pot, melt butter over medium-low heat. Add sweetened condensed milk and salt and stir until combined and warmed through.
Add mini marshmallows and stir until marshmallows are melted and smooth.
Sift matcha over top of melted marshmallow mixture and stir until evenly incorporated.
Remove from heat and pour over cereal, gently turning the mixture with a wide rubber spatula until it is completely coated.
Dump mixture into prepared pan and press into an even layer with a spatula or greased hands. You’ll need to press lightly to spread it into an even layer, but don’t compress it too much or your treats won’t be as soft.
Let sit for 30 minutes or so to completely cool, then lift treats out of pan onto a cutting board and cut into triangles or other desired shape.
To decorate, melt white chocolate or candy coating gently in the microwave or in a double boiler, then transfer to a piping bag or squeeze bottle fitted with a small round tip (you can also use a zip-top bag with the tiniest bit of the corner cut off). Drizzle over cut treats in a decorative pattern, then top with sprinkles for decoration as desired.
Treats can be stored lightly covered at room temperature or in the refrigerator for up to five days.