Before you begin, let's do some prep. Start by sifting together powdered sugar and corn starch.
Next, grease a 10" x 8" (this size can vary) casserole dish or pan with vegetable oil. Add a teaspoon or two of the powdered sugar mixture to a sieve and sift the mixture into the casserole dish. Set both aside.
In a small bowl, whisk together the hot water and matcha powder until the matcha has completely dissolved. Set aside to cool.
Pour a 1/2 cup water into the bowl of a stand-up mixer (or you can use a large bowl and a hand-mixer). Sprinkle the gelatin atop water and allow to sit for 10 minutes so the gelatin can bloom.
Meanwhile, in a medium saucepan, with a candy thermometer attached to its side, combine the granulated sugar, corn syrup, salt and remaining 1/4 cup of water. Heat the mixture over medium heat until the sugar has dissolved. Turn the heat up to moderately high heat and bring the mixture to a hard boil and cook for 1 minute, until the candy thermometer reaches 240 degrees F.
Lower the whisk attachment and turn it on low. Carefully add the boiling liquid to the gelatin mixture. Turn the mixer to high and beat for 8-10 minutes, until the mixture has doubled in volume and holds stiff peaks. Pour in the matcha/water mixture and beat for an additional minute or so until incorporated.
Pour the mixture into the prepared casserole dish or baking pan, smoothing out the top until it's evenly disbursed (it'll be sticky!). Dust the top with a tablespoon of powdered sugar/cornstarch mixture. Cover with foil or plastic wrap and allow to set until firm, about 4 hours or overnight.
Take a knife and run it around the edges of the casserole dish or pan and invert the marshmallow sheet onto a large cutting board, smacking the bottom of it, if needed. You also may need to use your fingers to loosen the marshmallow sheet from the casserole dish and gently glide it onto the cutting board. Using a sharp knife, cut the marshmallows into 1-inch cubes. Store the marshmallows in an airtight container for up to 1 week.
You know how to make s'mores: toast the shit out of your marshmallow, stick it on a graham cracker that has a piece of Hershey's chocolate on it and then smash it with another graham cracker. This is called heaven.