White Chocolate Mousse Cake

Ingredients

White Chocolate Mousse

• 1 1/3 cups good quality white chocolate chips (215g)
• 1 1/2 cups heavy cream or heavy whipping cream, cold – divided (360ml)
• 1/4 cup powdered sugar (30g)
• 1 tsp vanilla extract (4mL)
• pinch of fine salt (less than 1/8 tsp)

White Cake

• 1 cup or 2 stick unsalted butter, room temperature (226g)
• 3 cups granulated sugar (600g)
• 1 cup pasteurized egg whites from a carton or about 7 egg whites, room temperature (235g)
• 3 cups cake flour (360g)
• 2 1/2 tsp baking powder (10g)
• 1 tsp salt (6g)
• 1 1/2 cups full-fat sour cream, room temperature (380g)
• 1/4 cup vegetable oil (56 ml)
• 1 tsp vanilla extract (4ml)

Buttercream Frosting

• 2 cups unsalted butter, room temperature (452g)
• 7 cups powdered sugar (900g)
• 1 tsp vanilla extract (4ml)
• 1/2 tsp salt (3g)
• 1/3 cup heavy cream or whipping cream (75g)
• 1 cup white chocolate chips, melted and cooled (160g)

Tools and Equipment

• Stand mixer or electric hand mixer
• Whisk attachment
• Greaseproof Cake Board
• Spinning Cake Stand
• Icing Comb
• Large Offset Spatula
• Bench Scraper

Direction

White Chocolate Mousse Filling

1. Make the white chocolate mousse first, so it has time to chill. Or better yet, make this mousse in advance! It can be made 1-2 days ahead of time.
2. Add the white chocolate chips and 1/2 cup heavy cream in a medium-sized, heat-proof bowl. Heat for 60-75 seconds in the microwave, then let the mixture sit for a minute. Stir until combined with a rubber spatula. The white chocolate should be completely melted, and the mixture should be smooth. If you don’t have a microwave, you can also use a double boiler. Set aside.
3. Add 1 cup of heavy cream, powdered sugar, vanilla extract, and a pinch of fine salt in a large bowl or the bowl of a stand mixer fit with a whisk attachment.
4. Mix on medium-high speed until the whipped cream starts to gather in the whisk attachment and you can see texture around the edge of the bowl. The whipped cream should be thick and able to keep its shape. This usually takes about 2 1/2 to 3 minutes in my stand mixer.
5. Gently fold half of the whipped cream into the white chocolate cream mixture using a rubber spatula. At this point, the white chocolate mixture should have cooled down a bit and should still be fluid but shouldn’t be hot to the touch. Use the spatula to swoop around the sides of the bowl and pull through the center like I show in the video of this recipe. Fold in the remaining half of the whipped cream. Be careful not to over-mix the mousse, or it will become deflated and lose its structure.
6. Cover the mousse with plastic wrap and chill in the refrigerator for at least 2 hours.

White Cake Layers

1. Preheat oven to 325°F. Line and grease four, 7-inch pans or three, 8-inch cake pans with parchment rounds.
2. Add the unsalted butter and granulated sugar into the bowl of a stand mixer. Mix together on a high speed with a whisk attachment for a few minutes. Scrape down the sides of the bowl as needed with a rubber spatula. The mixture should become lighter in color as air is whipped into it.
3. Add in the room-temperature egg whites and mix at a medium speed until they’re incorporated.
4. Whisk together the cake flour, baking powder, and salt into a separate bowl.
5. Add half of the dry ingredients into the butter mixture and mix on a low speed until incorporated.
6. Add the sour cream, oil, and vanilla extract and mix at a low speed until combined. Scrape the sides and bottom of the bowl as needed.
7. Mix in the remaining dry ingredients on a low speed.
8. Divide the batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans to make sure my layers bake up to be the same height.
9. Bake for 37-40 minutes, or until a toothpick comes out with a few moist crumbs. Rotate the pans halfway through to help them bake evenly.
10. Allow the pans to cool for 10 minutes, then run an offset spatula around the perimeter of the pan to separate the cake from the pan.
11. Place the cake layers into the freezer for 45 minutes to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.
12. Use a serrated knife to level the tops and trim the sides to remove any caramelization.

White Chocolate Buttercream Frosting

1. While the cake layers bake and cool, make the white chocolate buttercream frosting.
2. Beat the butter on a low speed for 30 seconds with a paddle or whisk attachment until smooth.
3. Add in the vanilla extract and salt and beat on a low speed.
4. Slowly add in the powdered sugar and mix on a low speed. Add in the heavy cream halfway through to make the frosting easier to mix.
5. Mix in the melted and cooled white chocolate on a low speed and scrape down the sides and bottom of the bowl with a rubber spatula as needed.
6. Continue to mix until the ingredients are fully incorporated and the desired consistency is reached. 
7. If the frosting is too thick, add in additional cream (1 teaspoon at a time). If the frosting is too thin, add in more powdered sugar (a quarter of a cup at a time).
8. Stir by hand with a rubber spatula to make the frosting extra smooth, then place the frosting in a large piping bag. Seal the top of the bag with a rubber band or clip and cut an opening that’s about 3/4 inch wide at the base of the bag. Set aside.

White Chocolate Mousse Cake

1. Stack and frost the cake layers on a greaseproof cake board using a dab of frosting to help stick the first cake layer to the board.
2. Pipe a ring of buttercream around the perimeter of each cake layer, then spread an even layer of white chocolate mousse inside the ring. Use about 1/3 of the mousse between each layer if you have 4 cake layers. If you have 3 cake layers, use 1/2 of the mousse between each layer.
3. Repeat with the remaining cake layers. Once all the cake layers are stacked, spread a thin coat of frosting around the cake to fully cover the cake layers. If your layers are sliding around, insert a chopstick or wooden dowel into the center of the cake to make it more stable.
4. Smooth using a bench scraper, then chill the cake in the fridge (30 minutes) or freezer (10 minutes) until the frosting is firm to the touch.
5. Add a second, thicker layer of white chocolate frosting to the cake and smooth using an icing comb.
6. Decorate as desired! I chose to add some buttercream swirls piped with a Wilton 1M frosting tip along with a pretty sprinkle blend.