Marshmallows:

Line an 8 x 8-inch square tin with parchment, and spray generously with nonstick spray.

Pour 120 milliliters (1/2 cup) water into a large mixing bowl (if using a hand mixer), or the bowl of a stand mixer. Sprinkle powdered gelatin evenly over the water. Set aside.

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

Carefully pour hot syrup over gelatin. Mix on high speed for 2-3 minutes, until mixture begins to thicken. Add peppermint oil and green gel food coloring and continue to mix on high speed until very thick and voluminous, about 8-9 minutes more.

With an oiled rubber spatula, scrape marshmallow into prepared tin.

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

Cutting and Coating:

In a small bowl, whisk potato starch and confectioner’s sugar. Dust generously over a large chopping board. Turn marshmallow slab out onto dusted surface.

Sift top of marshmallow slab with more powder. With a well-oiled chef’s knife, cut into squares.

Toss cut marshmallows in additional powder, coating all sides evenly.

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