2. If you have a Cream Canoe Pan, spray it with non-stick cooking spray. If you do not, make your own molds as follows:
Tear off a large square of aluminum foil (about 12-inches square), then fold it in half one way, then the other, so you have a smaller square. Wrap the piece of foil around a spice bottle, closing the ends, and leaving the top open. Remove the bottle and place the foil form on a baking sheet or in a large baking pan. Repeat with 9 more pieces of foil to make 10 total molds. Spray the inside of the molds with non-stick cooking spray and set aside.
3. In a medium bowl, beat the egg whites on medium-high speed until stiff peaks form; set aside.
4. In a large bowl, combine the cake mix and water on low speed for 30 seconds, then on medium speed for an additional 2 minutes, making sure it's thoroughly blended. Using a rubber spatula, fold in ⅓ of the egg whites to lighten the batter. Then, gently fold in the remaining egg whites until completely combined.
5. Divide the batter evenly between the molds.
Bake for 30 minutes, or until the cakes are golden brown on top and a toothpick inserted into the center comes out clean. Using oven mitts and being careful not to get burned, turn the cakes out of the foil molds and place them right-side-up on a cooling rack. Allow to cool completely before filling.
6. Make the Filling: In a small bowl, stir together the hot water and salt until the sale has dissolved; set aside. In a large bowl, combine the marshmallow creme, shortening, powdered sugar and vanilla; beat on high speed until light and fluffy, scraping the sides of the bowl as needed. Add the salt solution and beat again on medium speed for 30 seconds to 1 minute, or until thoroughly combined.
7. Use a toothpick or skewer to poke three holes in the bottom of each cake, moving it around to create a sizable hole to fill with frosting. Use a piping bag and decorating tip or a plastic ziploc bag with a corner snipped off and pipe frosting into each of the holes, filling as much as you can without allowing the cake to crack.
Store snack cakes at room temperature in an airtight container, or wrap individually with plastic wrap.