Chocolate Raspberry Mousse Cake

Instructions

Chocolate Mousse Filling

1. Make the 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. Melt the chocolate chips by placing them in a medium-sized, heat-proof bowl. Heat in 20-30 second increments in the microwave, stirring after each increment until the chocolate completely melted and smooth. If you don't have a microwave, you can also use a double boiler.
3. In a small bowl, whisk together the warm water and cocoa powder. Pour into the melted chocolate and whisk together until smooth. The mixture should be somewhat thick. Set aside.
4. Add the 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.
5. Mix on medium-high speed until the whipped cream starts to gather in the whisk attachment and you can see texture around the edges 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.
6. Gently fold the chocolate mixture into the whipped cream using a rubber spatula. 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. Be careful not to over-mix the mousse, or it will become deflated and lose its structure.
7. Cover the mousse with plastic wrap and chill in the refrigerator for at least 2 hours.

Chocolate Cake Layers

1. Preheat oven to 350 F / 175 C. Grease and line four, eight-inch cake pans.
2. Sift or whisk together the dry ingredients (flour, sugar, dark cocoa powder, baking powder, baking soda and salt) into a large bowl.
3. Pour in the warm water, buttermilk, oil, eggs, and vanilla into a separate bowl and whisk together until fully incorporated.
4. Mix the wet ingredients into the dry ingredients just until they're incorporated into the batter.
5. Divide the batter evenly between the prepared cake pans, and bake for about 23-26 minutes or until a toothpick comes out with a few moist crumbs.
6. Remove the cake layers from the oven and let them cool in the pans for about 10 minutes. Gently run an offset spatula around the rim of the cake pans to loosen them, then flip them into wire racks to finish cooling.
7. Once cooled, use a serrated knife to level the top of each cake layer. If you are making these in advance, wrap and freeze them at this point.

Chocolate Buttercream Frosting

1. Beat the butter on a medium speed for 30 seconds with a paddle attachment until smooth. 
2. Add in the sifted dark cocoa, vanilla extract and salt. Mix on low until the ingredients are fully incorporated.
3. Slowly mix in the powdered sugar. Halfway through, add in the heavy cream to make the frosting easier to mix.
4. Mix in the cooled, melted dark chocolate on a low speed until the ingredients are fully incorporated and the desired consistency is reached. Cover with plastic wrap to prevent crusting and set aside.

Assembling This Chocolate Raspberry Mousse Cake

1. Stack and frost cake layers on a greaseproof cake board, using a dab of chocolate buttercream to help stick the first cake layer to the board.
2. As you assemble the cake, spread about 1/4 cup of raspberry jam on top of each cake layer. Spread about 1/3 of the chocolate mousse in an even layer on top of the jam. If desired, gently press about 2/3 cup of fresh raspberries into the mousse.
3. Repeat with the remaining cake layers. Smooth any overhanging mousse around the cake with a large offset spatula, then freeze the cake for about 10 minutes in the freezer. This will help the buttercream between the cake layers firm up and make the cake way easier to frost.
4. Carefully cover the cake with a thick, even layer of chocolate buttercream and use an offset spatula or the back of a spoon to make some cute swoops around the cake. This is an easy and stress-free way to decorate a cake that gives it a beautiful finished look.
5. Top with a semicircle of fresh raspberries and enjoy!