Preheat oven to 350 degrees. Spray or line a standard muffin tin.
Add cake mix to a large mixing bowl. Using a hand or stand mixer, mix in your water, oil, and egg whites on medium until well incorporated.
Add batter into the muffin tin until each is about 2/3 full. Bake for 18-20 minutes, or until a toothpick comes out clean. Allow to cool completely before adding the marshmallow fluff filling.
Directions for Marshmallow Fluff Filling:
In a medium mixing bowl, using a hand or stand mixer, cream the butter, and beat until light and fluffy. Mix in the powdered sugar, half and half and marshmallow flavoring until well incorporated.
Using the bottom of a spoon or other utensil, poke a hole in the middle of each cupcake, about ¾ down. Add the marshmallow fluff filling to the cupcake injector. Carefully inject the filling into the middle of each cupcake.
Directions for Frosting:
In a large mixing bowl, using a hand or stand mixer, cream the butter, and beat until light and fluffy. Mix in the vanilla extract. Mix in the marshmallow cream until smooth and creamy
Slowly mix in the powdered sugar, 1 cup at a time, while adding a bit of the half and half or milk in between each cup. Continue to mix for 1 minute or until creamy.
Evenly divide the frosting into 6 containers. Add in one of each color of the food coloring, 1-2 drops at a time to each container (red to one, orange to the next, etc.). Mix until desired color is achieved.
Add each color to its own small piping bag until you have 6 bags of frosting in a rainbow of colors. Tear off a sheet of plastic wrap about 18-22” long. Pipe a 1-2” wide strip of frosting up the plastic wrap. They should be about 12-15” long. Repeat until you have a strip for each color.
Now you are carefully going to roll the frosting, moving from side to side, starting with the red, so you have a log of rainbow frosting. Make sure it is nice and firm but don’t squeeze it, so they are mushed together! Twist the ends together.
Snip one twisted end off and insert into your large, prepared piping bag. You may want to squeeze a bit onto some plastic wrap until you see the rainbow colors come out before frosting the cupcakes. Frost your cupcakes and enjoy!