1/2cupbuttercream frostingor royal icing, see footnote
Spray a 9×13-inch cake pan with cooking spray.
Yellow cake marshmallow layers: Whisk together the gelatin and cold water in a small bowl. Let set until gelatin absorbs liquid.
In a saucepan with a candy thermometer inserted, combine sugar, 1/2 cup corn syrup, 1/2 cup water and salt. Place over high heat and bring to a boil. Meanwhile, pour the remaining 1/2 cup corn syrup in the bowl of a standing mixer fitted with the whisk attachment. Heat the gelatin in the microwave or on the stove-top until melted completely and pour into the mixing bowl with the corn syrup. Turn mixer on low and keep it stirring until the sugar syrup is ready.
When the syrup reaches 240°F, slowly pour it into the mixer bowl. Increase the speed to medium and beat for 5 minutes, then increase to medium-high and beat 5 more minutes. Finally, increase to highest speed and beat 1-2 more minutes, add cake batter flavor then beat again for 1-2 minutes. Mixture should be opaque and tripled in volume. Add yellow food coloring and beat until incorporated.
Pour the marshmallow into the prepared pan and let stand until set, about 2 hours.
Coating: Stir together the corn starch and powdered sugar in a bowl. Sift about 1/4 cup over a work surface.
Turn out the marshmallow onto the coating. Sift a little more powder over the surface of the marshmallow and cut out two 5-inch circles using a pastry ring. Cut leftover marshmallow into pieces, coat with powder, and save for snacking. Let the two yellow rounds stand uncovered while you make the pink marshmallow.
Grease a 6×3 inch round cake pan with cooking spray. Line the edges with parchment paper so that the paper extends 3 inches over the top edge of the pan (for a total height of 6-inches in the pan – see blog photo with ruler).
For the pink marshmallow, repeat the instructions for the yellow marshmallow exactly, except use 1 teaspoon butter flavor in place of the cake batter flavor, and use neon pink food color in place of the yellow.
Pour some of the pink marshmallow into the prepared pan, about 2-inches up the side. Press a yellow layer into the pink marshmallow until the yellow and pink layers are even. Top with more pink marshmallow and add another yellow layer. Finally, fill the parchment to the top with pink marshmallow.
Let the marshmallow stand uncovered in the pan overnight. When set, pull the marshmallow out of the pan by the parchment paper and dust it with the coating.
Cut the marshmallow into pieces using a sharp chef’s knife that has been warmed under hot water and wiped dry. Cut the marshmallow cake into four pieces, and then each piece into 2, giving a total yield of 8 pieces.
Decors: Lay each piece on its side on serving plates and pipe a buttercream border on the top and bottom edges of the slices. Cover with sprinkles and let stand until set.
If packaging these for favors, use royal icing that crusts to hard candy consistency for the buttercream borders.