In a large microwave-safe bowl, combine the homemade matcha marshmallows with butter, vanilla extract and sea salt.
Microwave for 20-25 seconds, until partially melted. (The marshmallow mixture will puff up).
Dump in the crisp rice, black sesame seeds (if using) and coat the mixture evenly with a spatula.
Transfer the matcha crispy rice into a parchment-lined baking tray (I used a 7.5" by 6" pan) and press down firmly.
Let the matcha crispy rice dry and set, uncovered in a draft-free location, at least overnight.
The next day, check to see it has set -- break off a clump -- if the mixture is still wet, preheat oven to 250°F/121°C and place it in for 20-30 minutes to completely dry out.
Let it completely cool and break off clusters with your hands and place into a dry, airtight container at room temperature for up to 1 week.