In a medium sauce pot, combine ½ cup water, 1 cup sugar, and corn syrup. Set over medium-high heat and cook to soft ball stage, 240°F. Line a 9-inch square pan with parchment paper, allowing for an overhang on all four sheets. Lightly grease parchment paper and set aside.
When the sugar mixture reaches about 190°F, bloom the gelatin. Place gelatin sheets in a bowl of ice water. Let sit for 5 minutes, or until sheets have softened. Remove sheets from the water and squeeze out any excess liquid. Place softened gelatin in a small bowl and microwave it at low power until it melts into a liquid. It takes about 10-15 seconds
In a small bowl mix together 1 Tablespoon matcha powder and 2 Tablespoon water. Set aside.
Once sugar mixture has reached 240°F. Remove from heat and let mixture cool slightly to 210°F. Pour into the bowl of a stand mixer fitted with a whisk attachment. Add liquid gelatin and matcha mixture. Whip on low-medium speed for a minute or two to get all the ingredients combined. Then increase the speed and whip until mixture is light and fluffy.
Use a lightly oil spatula to pour marshmallow mixture into the prepared pan. Spread the mixture into an even layer. Sift matcha sugar dusting powder all over the top of the marshmallow. Let it sit at room temperature for 5 hours (up to overnight).
Remove the giant marshmallow from the pan and place it upside down on a confectioners' sugar dusted surface. Peel off the parchment bottom of the marshmallow. As you remove the parchment, sprinkle the matcha sugar dusting powder over the exposed marshmallow.
Use a knife or sharp scissors to cut the marshmallows into your desired size and shape. Have the matcha sugar dusting powder on hand to coat the marshmallows as you cut.