Line an 8x8-inch pan with aluminum foil. Set aside.

Whisk together the gelatin and cold water in a small bowl and let soften for 5 minutes

Stir together the sugar, ¼ cup of the corn syrup, ¼ cup water and salt in a medium saucepan over high heat. Boil, stirring occasionally, until the temperature reaches 240F on a candy thermometer.

Meanwhile, pour the remaining ¼ cup corn syrup into the bowl of an electric mixer fitted with a whisk attachment. Microwave gelatin on high until totally melted, about 30 seconds. Pour into the mixer bowl, Set the mixer speed to low and keep it running.

When the syrup is 240F, slowly pour it into the mixer. Increase the speed to medium and beat for 5 minutes. Increase to medium-high and beat for 5 more minutes. Beat on the highest setting for 1-2 mins more and beat in the vanilla. The finished marshmallow will be opaque, white, fluffy and tripled in volume. (don't overwhip the batter or it will make folding in the cake mix a huge mess.) Sift in about half the cake mix over the marshmallow batter. Use a spatula to gently fold it in and then sift and fold in the rest. Fold in the sprinkles.

Pour the batter into the prepared pan, using an offset spatula to smooth it into the corners. Sift the coating over the top, reserving some for later. Let it set for 6 hours in a cool dry place.

Use a knife to loosen the marshmallow from the edges of the pan. Invert the slab onto a coating-dusted work surface and dust it with more coating. Use kitchen scissors to cut the marshmallows into pieces and dip the sticky edges into more coating, patting off the excess.