Make the Cake Layers

1. Prepare three 8-inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
2. Add the white chocolate and ¼ cup of buttermilk to a medium sized bowl. Heat in 20 second increments, stirring well between each, until melted and smooth. Set aside.
3. Combine the flour, baking powder, baking soda and salt in a medium sized bowl and set aside.
4. Add the butter, oil and sugar to a large mixing bowl and beat together until light in color and fluffy, about 2-3 minutes. Do not skimp on the creaming time.
5. Add the eggs one at a time, mixing until mostly combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
6. Add half of the dry ingredients to the batter and mix until mostly combined.
7. Slowly add the remaining cup of buttermilk and vanilla extract mix until well combined.
8. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
9. Slowly add the melted white chocolate mixture and gently fold it into the cake batter.
10. Divide the batter evenly between the cake pans and bake for 25-30 minutes, or until a toothpick comes out with a few crumbs.
11. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.

Make the Buttercream

1. When the cakes have cooled, make the buttercream. Add the white chocolate to a bowl and heat in 10-20 second increments, stirring well between each, until melted and smooth. Set aside.
2. Add the butter to a large mixer bowl and beat until smooth and creamy.
3. Slowly add half of the powdered sugar and mix until well combined and smooth.
4. Slowly add the white chocolate in 5-6 batches, mixing between each addition of it until well combined. It’s good to add smaller amounts at a time so that the white chocolate doesn’t clump.
5. Add 4-5 tablespoons of heavy cream and the salt and mix until well combined and smooth.
6. Slowly add the remaining powdered sugar and mix until well combined and smooth. Add additional water or milk, as needed to get the right consistency of frosting.

Make the Ganache Filling

1. To make the ganache, add the white chocolate chips to a medium sized bowl.
2. Add the heavy whipping cream to a microwave-safe measuring cup and microwave until it just begins to boil.
3. Pour the hot cream mixture over the white chocolate chips and let it sit for 2-3 minutes, then whisk until smooth. If it doesn’t melt really well, pop it back in the microwave for about 10 seconds, then whisk until smooth.

Build the Cake

1. To put the cake together, use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. See my tutorial for leveling and torting a cake for additional help.
2. Place the first layer of cake on a cardboard cake circle or a serving platter.
3. Spread about 1/2 a cup of buttercream into an even layer on top of the cake layer, then use the frosting to create a dam around the outer edge of the cake. See my tutorial on how to build a cake for more guidance. Spread about half of the white chocolate ganache into an even layer inside of the dam, then level out the dam and ganache so that it’s all an even layer.
4. Add another layer of cake on top of the ganache, then repeat the process of adding frosting and ganache.
5. Add the final layer of cake on top, then frost the outside of the cake with the remaining vanilla buttercream. Refer to my tutorial for frosting a smooth cake, if needed.

Decorate

1. To make the ganache, add the white chocolate chips to a medium-sized bowl.
2. Add the heavy whipping cream to a microwave-safe measuring cup and microwave until it just begins to boil.
3. Pour the hot cream mixture over the white chocolate chips and let it sit for 2-3 minutes, then whisk until smooth. If it doesn’t melt really well, pop it back in the microwave for about 10 seconds, then whisk until smooth.
4. Drizzle the white chocolate ganache around the edges of the cake, then fill in the center/top of the cake. See my tips on making a chocolate drip cake.
5. To finish off the cake, use the remaining buttercream to pipe swirls around the outer edge of the cake. Top each swirl with a white chocolate truffle if you’d like.
6. Keep the cake in the fridge until you’re ready to cut and serve it.