Grease a 9-inch square baking pan with butter or use parchment paper.
In a large pot, melt the butter over low-medium heat.
Add the marshmallows, condensed milk and a tiny pinch of salt (optional) and stir until melted, about 5 minutes. Turn off the heat and add in the matcha powder and vanilla extract, stir until completely combined.
Add the cereal and quickly stir until evenly coated and additional mini marshmallows. Immediately press into the greased pan. *Pro Tip: Use the butter wrapper to press down without sticking.
Let cool for at least 10 minutes before flipping out onto a cutting board and slicing into squares. The treats can be stored in an airtight container for up to 4 days at room temperature.