Chocolate Mousse-Filled Chocolate Layer Cake

The cake:

1. Preheat the oven to 325 degrees F.  Grease two 9-inch round cake pans and line the bottoms with parchment paper.
2. Place the chopped chocolate in a medium bowl and pour in hot brewed coffee. Stir until the chocolate is melted. Set the mixture aside.
3. In a large bowl, sift together sugar, flour, cocoa powder, baking soda, baking powder, and salt. Mix well with a whisk.
4. In the bowl of a mixer (or just a large bowl if using a hand mixer), beat eggs until they thicken slightly, about 3 minutes.  Gradually mix in canola oil, buttermilk, vanilla, and chocolate/coffee mixture until the mixture is well combined.
5. Gradually add the sugar mixture, and beat on low speed until combined. Avoid beating on medium or high speed so that there is not too much air in the batter. Do not overmix or the cake will become tough.
6. Pour the batter into the prepared pans and bake until a tested inserted in the center comes out clean, about 50 to 65 minutes.  Place the pans on a rack and let the cakes cool in the pans for 20-30 minutes. Once cool, invert the cakes onto the rack. Carefully remove the parchment paper.
7. I recommend making the cake layers earlier in the day or the day before. Once the layers are completely cool, wrap well in plastic wrap and refrigerate. If you have the opportunity, freeze the layers for about 1 hour. This will allow the cake to become slightly firm, which will make the layers much easier to split later.

The mousse:

1. In the bowl of a mixer, using the whisk attachment (or using a hand mixer), beat 2 ½ cups of heavy whipping cream just until stiff peaks barely form (i.e.,stiffer than soft peaks). 
2. Place chopped milk chocolate in a medium bowl.Heat 1 cup of heavy whipping cream in a small saucepan, just until bubbles start to form on the surface. Pour the cream over the milk chocolate. Using a whisk, stir until the chocolate is completely melted.Let the mixture cool for 10-15 minutes.
3. Pour the chocolate mixture into the whipped cream and fold until the two are well combined.

Assembling the cake:

1. Cut the two layers in half horizontally, using a large serrated knife.  Place one of the cake layers on a serving dish or a cake circle and smooth ¾ cup of the mousse on the top of the cake layer.  Repeat with the rest of the layers, so that there are 4 layers of cake and 3 layers of filling.
2. To make the cake as even as possible, place a serrated knife vertically against the cake and cut off any uneven parts. This is not essential, but it makes fora nicer presentation.
3. With a small amount of the mousse mixture, cover the cake with a crumb coating (alight coating of icing). This will help to seal in the crumbs so that you don’t end up with a bunch of crumbs in your final icing layer. Cover and chill for 30minutes.
4. Using an offset spatula, ice the cake with the rest of the mousse mixture. On the large holes of a box grater, grate about 4 ounces of semisweet or bittersweet chocolate. Cup some of the shavings into the palm of your hand and gently press the chocolate onto the side of the cake. Continue with the rest of the shavings. Cover and refrigerate until ready to serve.