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best buttercream frosting

Harry Potter Cake

This Harry Potter Cake is to celebrate Harry Potter’s birthday! Hagrid’s baking skills leave much to be desired but this Harry Potter Birthday Cake is so tasty you won’t have any leftovers!
Course Dessert
Cuisine American
Prep Time 10 minutes
Cook Time 25 minutes
Total Time 35 minutes
Servings 8 slices
Author Lisa


For the Cake

  • 1 box white cake mix I used Pillsbury brand
  • ½ cup vegetable oil
  • 2 tbs vanilla
  • ¾ cup sour cream
  • ¾ cup heavy cream
  • 4 egg whites

For the Best White Butter Cream Frosting

  • 1 cup butter cold and cut into chunks
  • 5 cups powdered sugar
  • 1 tbs vanilla
  • 5 tbs heavy cream
  • Pinch of salt
  • Neon purple and green food coloring

Extra Items Needed for Decorating

  • 6 inch cake pans I use 3 at a time
  • 1 6 in carboard cake round optional
  • 1 8 in carboard cake round optional
  • Cake Decorating Turntable optional


  • 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. I like to use my fingers to make sure the butter gets in all seams.
  • Mix together, either by hand or in a stand mixer, the oil, vanilla, sour cream, heavy cream and egg whites. Then mix in the boxed cake mix. Divide the batter between the 3 cake rounds.
  • 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.

To make the frosting:

  • In a stand mixer, whip your butter chunks on medium high speed until the butter is light and fluffy.
  • Add in the powdered sugar 1 cup at a time, letting it mix into the butter completely.
  • Add in the vanilla, heavy cream, and pinch of salt, allowing the ingredients to incorporate fully.
  • Measure out approximately 1.5 cups of frosting and mix in the green food coloring to it. Add neon purple color to the remaining frosting and mix in completely.
  • Refrigerate both frostings for 15-20 minutes to allow it to thicken up before using to decorate. If the frosting is too thick after refrigerating, it will soften up quickly at room temperature. So just be patient. If the frosting is too runny, add more powdered sugar to thicken it.

To decorate the cake:

  • On the cake decorating turntable, if using, place an 8 inch cardboard round (if using), then place a 6 inch round (if using) on top of that. Taping the 6 in to the 8 inch round is also advised. If not using any of the above just place the cake on what you are wanting to serve the cake on, but remember it has to fit in the freezer for a little while!
  • Add a spoonful of frosting to the center of the 6 inch round (or cake serving plate) and place your bottom cake layer on top, centered on the 6 inch cardboard round.
  • Add a crumb coat layer of frosting over the entire cake using the purple frosting*. A crumb coat layer is a thin layer of frosting that helps keep all the crumbs stuck to the cake and allows you to frost the cake really nicely later on. The crumb coat is not supposed to be pretty!
  • Stick your crumb coated cake in the freezer for 10 minutes.
  • After 10 minutes, remove the cake and frost with the final layer of purple 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.
  • Then using a writing piping tip, piping bag and green frosting, pipe on HAPPEE BIRTHDAE HARRY.
  • Slice, enjoy and watch all the movies!


*This will make three 6 inch cakes. So either frost the extra two cakes and eat them or double wrap them in saran wrap and freeze them for up to a few weeks. *In the picture I crumb coated this cake in white buttercream, then died the remaining frosting purple and green. In hindsight I would recommend using purple for a crumb coat in addition to the final frosting. But either way it is delicious!