Marshmallow Slab

Line an 8×8-inch / 20 x 20-centimeter square tin with parchment, and coat generously with oil. (See this recipe for the best lining technique!) Set aside until ready to use.

Pour 105 milliliters (1/2 cup) water into a large mixing bowl (if using a hand mixer), or the bowl of a stand mixer. Sprinkle the gelatin evenly over the water, and set bowl aside.

Combine sugar, corn syrup, remaining water, and salt in a large saucepan. Bring mixture to a boil and boil, without stirring, until syrup reaches 240° F / 116° C on a candy thermometer.

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

Add rainbow nonpareils and mix on low for 15-30 seconds, just long enough to incorporate the chocolate into the mixture.

Using an oiled spatula, scrape marshmallow mixture into prepared tin. Loosely cover with a well-oiled sheet of aluminum foil. Allow to set until firm, at least 4 hours or overnight.

Cutting and Coating

In a small bowl, combine potato starch and confectioner’s  sugar.

Cover a large chopping board with a sheet of parchment, and dust generously with starch-sugar mixture.

If you want to coat one side of your marshmallow slab in sprinkles, do it now. It’s much easier (and less messy!) to pour the sprinkles on top of the marshmallow mixture while it’s still in the tin. The top of the slab will also be the stickiest, since it hasn’t been coated in oil.

Remove the foil covering and pour a generous amount of sprinkles on top of the slab. Use your hands to spread, making sure the marshmallow layer has been completely covered. (Note: If you’re planning to skip the sprinkle layer, you can simply proceed to the step below.)

Lift marshmallow slab out of tin. Peel away the parchment, and place onto dusted surface, sprinkle side up. (If not using sprinkles, sift top of slab with more of the starch-sugar mixture.)

Use a well-oiled knife to cut the slab into squares. You can make small squares (six rows of six works well, for 36 average-sized marshmallows) or cut 16 oversized marshmallows as I’ve done here. Or, use an oiled metal biscuit cutter to cut the slab into fun shapes–it’s really up to you!

Toss each marshmallow in additional starch-sugar mixture, making sure all sticky sides are coated.

Marshmallows will keep well for several weeks when stored in an airtight container at room temperature.