Preheat oven to 350°F. Line two cookie sheets with parchment paper.
In a large mixing bowl, beat cake mix, water, oil, and eggs on medium speed for 2 minutes, scraping bowl occasionally.
On a parchment lined baking sheet, drop heaping tablespoons of cake batter two inches apart.
Bake for 8 to 10 minutes or until toothpick comes out clean in the center of a cake.
Allow cakes to cool on parchment for 10 minutes on a wire rack.
In medium bowl, beat shortening and butter with electric mixer on high speed 3 minutes.
Reduce speed to low and add granulated sugar, mint extract, food color and milk.
Add powdered sugar 1/2 cup at a time until smooth.
Spread a small spoonful of mint frosting between two cakes and decorate the edges with Lucky Charms® marshmallows.