Line your baking sheet with parchment paper and preheat your oven to 350 degrees.
Start mixing your cake mix and its ingredients in a large mixing bowl. Once the cake batter has formed, add cocoa powder and mix until evenly incorporated.
Add gel green food coloring and mix until well distributed.
Using a spatula, spread your cake batter onto your lined baking sheet. Baking time will vary, but it will be about 12-16 minutes (considerably less than standard cake timing).
To make the buttercream, beat together butter, sugar and milk on medium speed. Gradually increase the speed to high.
Once the frosting is light and fluffy and can form semi-stiff peaks, scoop it into a piping bag.
Remove cake from oven and allow to cool completely. Using your cookie cutter, cut out 4-leaf clover shapes set aside cut off bits for other use or toss.
Flip over each shaped cake (they look better from the underside and are easier to frost this way).
Frost half of your shaped cakes and then sandwich with another unfrosted piece.