Marshmallow Slab:

Line an 8 x 8-inch square tin with parchment in a criss-cross pattern, allowing a 1-inch overhang on all sides. Spray with nonstick spray. Set aside.

To a large mixing bowl or the bowl of a stand mixer, add 120 milliliters (1/2 cup) cold water. Sprinkle with the powdered gelatin. Set aside.

In a large, heavy-bottomed stock pot, stir together granulated sugar, corn syrup, remaining water, and fine sea salt. Bring to a boil. Boil, without stirring, until syrup reaches 240° F / 116° C on a digital thermometer.

Carefully pour hot syrup over gelatin mixture. Mix on high speed until very thick and voluminous, about 10 minutes.

With a fine mesh strainer, sift in in the matcha powder. Mix briefly to combine.

With a well-oiled rubber spatula, scrape marshmallow mixture into prepared tin.

Loosely cover with a well-oiled sheet of aluminum foil. Set aside and allow to set at room temperature until firm, at least 4 hours, or overnight.

Cutting and Coating:

In a small bowl, whisk together confectioner’s sugar, cornflour and matcha powder.

Dust a large chopping board with the cornflour-matcha mixture. Turn marshmallow slab out onto dusted surface and sift with more of the mixture.

With a well-oiled knife, cut slab into six rows of six, creating 36 squares.

Toss each square in additional cornflour mixture, coating all sides evenly.

Store matcha marshmallows in an airtight container at room temperature for up to 1 month.