• Preheat oven to 325°F. Prep your cake rounds by covering the inside of the pans with a light layer of butter and then dusting with flour, or use a nonstick baking spray like baker's joy.
• Mix together, either by hand or in a stand mixer, the oil, water and egg whites. Then mix in the boxed cake mix. Pour batter in cake pans you are using.
  1 box white cake mix, ⅓ cup vegetable oil, 1.25 cup water, 3 egg whites
  
• Bake for 25 minutes or until a toothpick inserted comes out clean. Make sure to bake all the cakes on the same oven rack so they bake evenly.
• Let the cakes cool for 5 minutes then flip upside down onto a wire cooling rack and remove the pans. They should come out easily.
• Let the cakes cool completely before frosting. Level the cakes after cooling if need be.
  

To make the frosting:

• In a stand mixer, whip your butter on medium high speed until the butter is light and fluffy.
  1 cup butter
• Alternate between adding in the powdered sugar 1 cup at a time and the heavy cream 1-2 tbs at a time, letting them mix into the butter completely.
  5 cups powdered sugar, 5 tbs heavy cream
• Add in the vanilla and a pinch of salt, allowing the ingredients to incorporate fully.
  1 tbs vanilla, Pinch of salt
• Measure out approximately 1 cup of frosting and mix in the green food coloring to it. Add the pink color to the remaining frosting and mix in completely. If the frosting is too thin, add more powdered sugar ¼ cup at a time, if too thick add a more heavy cream 1 tbs at a time.
  Pink and Green food coloring

To decorate the cake:

• Add a spoonful of frosting to the center of cake plate and place your cake centered on top.
• Add a crumb coat layer of frosting over the entire cake using the pink frosting*. A crumb coat layer is a thin layer of frosting that helps keep all the crumbs stuck to the cake. The crumb coat is not supposed to be pretty!
  
• Stick your crumb coated cake in the refrigerator or freezer for 10 minutes to allow it to harden.
• After 10 minutes, remove the cake and frost with the final layer of pink frosting, This does not have to be pretty since you want it to look like Hagrid made it! Using a butter knife make several lines so it looks like the cake was pieced together, similar to what it looked like in the movie.
• Then using a writing piping tip, piping bag and green frosting, pipe on HAPPEE BIRTHDAE HARRY. You can also just cut the tip off a zip loc baggie and use that.