Start by preheating your oven to 350 degrees or whatever the boxed cake mix calls for.
Combine your batter as instructed as box dictates
Add in the food coloring (go heavy, the color lightens as it cooks).
Continue to follow the box instructions regarding mixing times and when you are done, spoon the batter into the cupcake tins which have either been coated in non-stick spray or filled with cupcake liners.
Fill the tins 2/3 of the way full.
Bake as the box instructs and when done, remove from the oven and let sit 2 minutes before removing the cupcakes to a cooking rack.
Let cupcakes cool completely before frosting.
Once your cupcakes are cool, make your frosting. In a large bowl and using an electric mixer, whip your butter until creamy.
Add in your powdered sugar, vanilla, cream and salt.
Mix to combine.
Add in your green food coloring (and a bit of yellow too, if you have it) and then mix to combine. Spoon your frosting into a piping bag or a storage bag with the corner snipped off and frost your cupcakes.