a candy thermometer; a 9-by-13-inch cake pan; a 6-by-3-inch round cake pan; a 5-inch pastry ring; a soft-bristle kitchen-dedicated art brush; a piping bag fitted with a large French pastry tip
For the yellow marshmallow "cake" layers: Coat a 9-by-13-inch cake pan with cooking spray.
Whisk together the gelatin and 1 cup cold water in a small bowl. Let set until the gelatin absorbs the liquid.
In a saucepan with a candy thermometer attached, combine the sugar, salt, 1/2 cup of the corn syrup and 1/2 cup water. Place over high heat and bring to a boil.
Meanwhile, pour the remaining 1/2 cup corn syrup in the bowl of a stand mixer fitted with the whisk attachment. Heat the gelatin in a microwave or on the stove until melted completely. Pour into the mixing bowl with the corn syrup, turn the mixer on low and keep it stirring until the sugar syrup is ready.
When the syrup reaches 240 degrees 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 the highest speed and beat 1 to 2 more minutes. Add the butter flavor, then beat again for 1 to 2 minutes. The mixture should be opaque and tripled in volume. Add the golden yellow food coloring and beat until incorporated. Fold in the rainbow sprinkles. Pour the marshmallow into the prepared pan and let stand until set, about 2 hours.
For the marshmallow coating: Stir together the cornstarch 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 of the coating over the surface of the marshmallow and cut out two 5-inch circles using a pastry ring. Cut the leftover marshmallow into pieces, coat with more of the coating and save for snacking. Let the 2 yellow rounds dry (cure) for 4 hours.
For the white marshmallow "frosting:" Grease a 6-by-3-inch round cake pan with cooking spray. Line the edges with a double thickness of 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). Lightly coat the paper with cooking spray.
Repeat the instructions for the yellow marshmallow exactly, except use 1 teaspoon vanilla flavor in place of the butter flavor.
Pour some of the white marshmallow into the prepared pan, about 2 inches up the side. Press a yellow layer into the white marshmallow until the yellow and white layers are even. Top with more white marshmallow and press in the second yellow layer. Fill with the remaining white 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.
For the decors: Using a soft-bristle kitchen-dedicated art brush, coat the bottom half of the cake with corn syrup or piping gel. Pat rainbow sprinkles onto the sides of the cake.
Place the buttercream in a piping bag fitted with a large French pastry tip. Pipe 1.5-inch mounds of frosting around the top edge of the cake, pulling each mound of frosting into a peak. Immediately cover with rainbow sprinkles.
Cut the marshmallow into 4 pieces using a sharp chef's knife that has been warmed under hot water and wiped dry. Then cut each piece into 2, for a total of 8 pieces.