Preheat the oven to 350 F and line your baking tray with cupcake liners; set aside.
In a large bowl add the wet ingredients (water, oil, and eggs). Whisk the ingredients until the mixture is combined.
Slowly add the cake mix to the batter until just incorporated. It's okay if there are a few lumps.
Scoop out the batter using an ice cream scoop, and fill the liners about 2/3 full.
Bake for 15-19 minutes. Use a toothpick to insert in the cupcake to make sure it is done and nothing is left on the toothpick. Let the cupcakes cool for 5 minutes in the baking tray before transferring them to a wire rack to cool completely while you prepare the frosting.
In a large bowl add the butter. Beat on medium speed for 3-7 minutes. The longer you beat, the fluffier your frosting will be.
Add the vanilla extract. Mix for 1-2 minutes on medium speed until fully incorporated.
Add half of the powdered sugar. Mix on medium speed for 3 minutes. Scrape the sides of the bowl. Add the remainder of the powdered sugar.
Add the milk 1 Tablespoon at a time, if needed, to achieve the desired icing consistency.
Add the green food coloring a toothpick amount at a time. Mix on medium speed. Keep adding more coloring until you reach the green you want to achieve.
Use any tip for your frosting bag. Fill up the bag with frosting. Pipe the frosting onto the cupcake in a circular motion.
Add a red heart sprinkle to each cupcake.
Notes
Add 1/2 cup of cream cheese to the frosting if you want a creamier frosting. Eliminate 1/2 cup of the butter if you add the cream cheese.
Want to dye the batter green also? I think that would be fun and easy to do since you already have green food coloring for the icing. I'd do a darker green than the icing personally!
Add peppermint extract to the frosting for a minty flavor. You could make the cupcakes chocolate too (like we did for our Christmas tree cupcakes).