• Please read through the entire recipe first, as there are a lot of steps.

Chocolate Mousse Filling

• Before you prepare the mousse, pull all of the ingredients out for the cake layers that need to be room temperature (butter, eggs, coffee, milk, and sour cream).
• Pour 2 cups (480 grams) of heavy whipping cream into a measuring cup. Pour ¾ cup (180 grams) of that 2 cups into a separate cup/bowl, and put the 1 ¼ cups (300 grams) of heavy cream back into the fridge. You will use this later.
• Chop your chocolate and set aside.
• Fill a sauce pan with two inches of water and bring to a simmer over medium heat.
• In a medium-sized heat proof bowl, whisk together the egg yolks, granulated sugar, and salt until smooth. Place the bowl on the pot over the simmering water and continuously whisk while the mixture heats up. It should get to about 160F, although you don’t actually need to use a thermometer if you don’t have one. This should take about 4 minutes on medium heat.
• Meanwhile, microwave the ¾ cup (180 grams) of heavy cream that you set aside for 30-45 seconds. It should be luke warm. Once the egg mixture has reached 160F or it has been 4-5 minutes, slowly pour the warmed heavy cream into the eggs while continuously whisking. Keep whisking the mixture for another 2 minutes. Remove the bowl from on top of the saucepan, add the vanilla extract, and stir to combine.
• Add the chopped chocolate to a microwave safe bowl and microwave it in 30 second increments, stirring in between each increment until completely melted and smooth. You can also do this over the stove using a double boiler.
• Add the melted chocolate to the egg mixture and whisk until completely combined. Set aside.
• Remove the 1 ¼ cups (300 grams) of heavy cream you put in the fridge and pour it into the bowl of a stand mixer fitted with the whisk attachment or a large bowl if using a handheld mixer. Beat the heavy cream on medium-high speed for 3-4 minutes until medium peaks form. Medium peaks are in between soft peaks and stiff peaks - so not fully formed into whipped cream yet, but not super soft either.
• Pour the chocolate/egg mixture into the whipped cream and fold them together using a rubber spatula until combined. Avoid over mixing, as the whipped cream can lose all the air you just whipped into it and deflate.
• Pour the mixture into a large bowl and place in the fridge for 2-3 hours or so until set.

Dark Chocolate Cake

• While the mousse is setting, make the chocolate cake. Preheat oven to 325F and line three 8 inch cake pans with parchment paper.
• Finely chop the dark chocolate and set aside.
• In a medium-sized bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
• In the bowl of a stand mixer fitted with the paddle attachment, or a medium-sized bowl if using a handheld mixer, beat the butter and oil together until combined and completely homogeneous - this will only happen if the butter is completely at room temperature. About 2-3 minutes on high speed.
• With the mixer on low, slowly stream in the granulated sugar, then turn it up to medium-high speed and beat until light and fluffy. About 1-2 minutes, scraping down the sides and bottom of the bowl as necessary.
• Then, with the mixer on low, add the eggs in, one at a time, scraping down the sides and bottom of the bowl and mixing well in between each addition. Add the vanilla extract and mix on low until combined.
• Add half of the dry ingredients and mix on low until just combined. Add half of the milk, half of the coffee, and half of the sour cream and mix on low until just combined. Repeat this with the remaining halves of dry ingredients, milk, coffee and sour cream. Do not over-mix!
• Gently fold in the finely chopped chocolate in three additions.
• Separate the batter equally between the prepared pans (about 520 grams per pan) and spread it into an even layer. Bake the cakes for 25-32 minutes until a toothpick inserted into the center of the cake comes out clean. The cake will be slightly pulling away from the edges of the pan. Let the cakes sit in the pan for 10-15 minutes, then remove them onto a cooling rack to cool completely.

Milk Chocolate Ganache

• While the cakes are baking/cooling, make the ganache. Finely chop your chocolate and put it in a heat proof bowl. Set aside.
• Warm the heavy cream over the stove on medium heat until gently simmering and bubbles form at the edges of the pan. Pour the warmed heavy cream over the chopped chocolate and let sit for 3-4 minutes. The warmed heavy cream will melt the chocolate.
• Gently stir the cream and chocolate together until completely combined and smooth. Let the ganache cool at room temperature, stirring every 20 minutes or so, so the consistency remains the same throughout. It will thicken up to a buttercream texture.
• Cover tightly with plastic wrap until ready to use.

Chocolate Mascarpone Frosting

• In the bowl of a stand mixer fitted with the paddle attachment, or a large bowl if using a handheld mixer, beat the mascarpone and butter together on medium-high speed until combined, smooth and fluffy, and no more lumps exist. About 2 minutes.
• Add the vanilla extract, cocoa powder, one cup of the powdered sugar, and salt, and beat until combined - starting at low speed so the sugar doesn’t fly out of the bowl, then bumping it up to high speed. Continue to add one cup of powdered sugar add a time and repeat. Scrape down the bottom and sides of the bowl as necessary.
• If needed, add the milk then mix until combined. Once everything is combined, beat on medium speed for 2 minutes until fluffy. You may need to add more powdered sugar (to thicken) or more milk (to thin) depending on desired consistency.

Assemble

• Once the cakes are completely cool, we can begin to assemble. Cakes are easier to assemble when they are slightly chilled, so feel free to put the cakes in the fridge for 20 minutes prior to assembling.
• If the cakes have formed a dome top, use a serrated knife or cake leveler to carefully level off the cakes.
• Put the mousse, ganache, and frosting in their own piping bags. This is not necessary, but I find it makes assembly much easier.
• Take the first cake layer and pipe a ring of the chocolate mascarpone frosting along the edge. Pipe about ⅓ of the thickened milk chocolate ganache in that ring, and smooth it into an even layer.
• Pipe the chocolate mousse on top of the ganache and spread into an even layer. Note: this recipe yields a lot of chocolate mousse, enough to put some in between each cake layer AND enough to dollop a bit of chocolate mousse on each individual slice of cake. So don’t feel like you need to use all of the chocolate mousse when assembling the cake.
• Place the second cake layer on top. Repeat the same process as above - pipe a ring of chocolate mascarpone frosting, spread another ⅓ of the ganache, then some of the mousse.
• Place the final cake layer on top. At this point, I like to put the cake in the fridge for 30 minutes so the cake layers and filling have time to firm up and set. This will make it easier to frost the rest of the cake - if the filling is warm, the cake layers may be unstable and slide all over the place.
• After the 30 minutes, make a crumb coat. Remove the cake from the fridge and frost the tops and sides with a thin layer of frosting. Place it back in the fridge for 30 minutes. This will lock in the crumbs so they don’t get mixed in with the frosting while you are decorating.
• Use the remaining frosting to frost the cake completely. Spread the final ⅓ of ganache on top of the cake. If it has thickened too much, microwave it for 5 seconds to loosen it up. If you have leftover frosting, use a star tip to pipe icing on the tops of the cake in any design you like!
• Top the fully assembled cake with mini chocolate chips or chocolate sprinkles. Slice and enjoy! Dollop some of the leftover mousse on the cut cake slices.
• Store the cake in an airtight container.