Grease an 9×13-inch pan with shortening, using a paper towel to rub it lightly and evenly onto the bottom, sides and edges of the pan. Set aside.

Put the gelatin sheets into a medium microwave-safe bowl and fill it with very cold water, adding a few ice cubes to keep it cold. Let the sheets soak for about 10 minutes.

Place the sugar, 1/2 cup corn syrup and 1/2 cup water in a medium saucepan and stir together. Clip a candy thermometer onto the pan, and place it over medium-high heat. Bring it to a boil, stirring the mixture frequently–you are looking for it to eventually hit a temperature of 235-240 degrees (soft ball stage).

Meanwhile, pour the remaining 1/2 cup corn syrup into the bowl of an electric mixer fitted with the whisk attachment. By this point, the gelatin sheets should be very soft–drain them well and give them a quick wringing out, and place them back in the microwave-safe bowl. Microwave on high until the gelatin is completely melted, about 30 seconds. Turn the mixer on low, and very slowly pour the melted gelatin into the corn syrup. Keep the mixer running while you check the sugar syrup.

Once the syrup reaches 235-240 degrees, pull it from the heat. Carefully transfer the syrup to a large, heatproof measuring cup or a similar vessel with a spout for easy pouring. Turn the mixer up to medium speed and slowly pour the sugar syrup into the gelatin mixture. When all the syrup has been added, crank the speed up to medium-high and let it go for about 6 to 7 minutes–the mixture should turn white and fluffy. Add the vanilla extract, almond extract, as much food coloring as desired (if using. I used "Rose" gel food coloring by Wilton, about an 1/8 teaspoon), and salt and increase the speed to its highest setting for 1 more minute. Sift in half of the cake mix, and turn the mixer on low to let it combine. Once incorporated, sift in the other half and mix in. Add in about half of the sprinkles and combine. Don't stir too much - you don't want the colors of the sprinkles to streak.

Pour the marshmallow into the prepared pan and use an offset spatula spritzed with a bit of cooking spray to nudge it into the corners and smooth the top. Usually, they settle themselves pretty well and I don’t have to spread them much. Tap the pan on the counter a few time to get rid of air bubbles. Sprinkle the rest of the sprinkles evenly over the top of the marshmallow. Sift confectioners’ sugar evenly generously over the top. Let sit for about 6 hours or until firm.

Use a knife to loosen the marshmallow from the edges of the pan and invert it onto a confectioners’ sugar-dusted work surface. Dust the marshmallow slab with more confectioner’s sugar and cut into whatever size pieces you wish (a pizza cutter works great here). Dip the sticky edges of the marshmallows in more confectioners’ sugar, patting off the excess. Store in an airtight container for up to 1 week.