Preheat oven to 350 degrees F.
In a large bowl combine the cake mix, instant pudding, sour cream, and vegetable oil. Stir together until the ingredients just come together.
Add the eggs 1 at a time, mixing after each egg until it is incorporated into the batter.
If you are using food coloring add it now. Start with 2 drops of green food coloring, mix well and then add more if desired. I like to keep it a very pale shade of green.
Prepare a 9x13 metal baking pan by lightly spraying with nonstick spray and then lining the bottom with a piece of apartment paper.
Pour the batter into the pan.
Place the pan in the oven and bake for 25-30 minutes, or until a toothpick stuck in the center comes out clean.
Remove from the oven and let cool completely.
While the cake cools make the buttercream frosting. Add butter and powdered sugar to the bowl of your mixer and using the whisk attachment mix it together. Start slow so the powdered sugar doesn’t go everywhere. Slowly move to a higher speed to whip the frosting and make it light and fluffy.
Add the vanilla and continue to mix. If you want to color your frosting add 1-2 drops of green food coloring now.
You want a light, smooth texture to your frosting. If the frosting is still too firm add the heavy cream 1 teaspoon at a time, mixing well in between each, stopping when you reach the right consistency. Note: It is possible you will not need to add any cream.
When the cake is completely cooled use a knife or a spatula to frost the cake.
Sprinkle the top with the chopped pistachios (optional).
Serve and enjoy