Marshmallows:

Line an 8×8-inch square tin with parchment in a criss-cross pattern, allowing a 1-inch overhang on all sides. 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, combine granulated sugar, corn syrup, remaining water, and fine sea salt. Bring to a boil and boil, without stirring, until syrup reaches 240° F (116° C) on a candy thermometer.

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

Add the rainbow nonpareils and mix on low speed for 15-30 seconds, or until just incorporated.

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

Pour rainbow nonpareils over marshmallow, completely covering the top. With your hands, pat sprinkles to adhere.

Loosely cover tin with an oiled sheet of aluminum foil. Allow to sit at room temperature until set, at least 4 hours, or overnight.

Cutting and Coating:

In a small bowl, whisk potato starch and confectioner’s sugar. Generously dust a large chopping board with the powder.

Lifting by the parchment, carefully remove marshmallow slab from tin. Peel away parchment and arrange on dusted surface, sprinkle side up.

With a sharp, well-oiled chef’s knife, cut the slab into squares. You can cut 6 rows of 6, for 36 average-sized marshmallows, or 16 oversized marshmallows.

Toss each marshmallow in additional starch-sugar mixture, coating all of the sticky sides.

Store in an airtight container at room temperature for up to 3 weeks.