Triple Chocolate Mousse Cake

Bake #26 for 2025: Triple Chocolate Mousse Cake by Cook’s Illustrated

Wow wow wow! This 3-layer cake is as delicious as it is impressive on a plate. This recipe comes from the fantastic Desserts Illustrated cookbook by Cook's Illustrated. It's one of my favorite baking cookbooks and well worth a check out from your local library or adding to a wishlist here. While this is a little bit more involved as far as bakes goes, each indiviual layer is not really that complicated. If you've never done flourless chocolate cake or mousse, it's an excellent opportunity to try out both (bonus: two different types of mousse) and full of great baking techniques to practice. It's naturally gluten-free and will feed a small crowd. I took some to a neighbor, who later texted me saying, "5 stars! ⭐⭐⭐⭐⭐ for the mousse cake!!" Can't get a better endorsement than that.

It's a substantial recipe, but this was too delicious to not share, so I've typed it up with a few small notes below. Just work one layer at a time and in a few hours, you'll be serving up this ultra-light, decadent, and impressive looking dessert!

Ingredients:

Bottom Layer:

• 6 Tbsp (85g) unsalted butter, cut into pieces
• 7 oz (200g) bittersweet chocolate*, chopped fine
• 1/2 tsp instant espresso powder
• 4 large eggs, separated
• 1-1/2 tsp vanilla
• Pinch cream of tartar
• Pinch table salt
• 1/3 cup (71g) packed light brown sugar, divided

Middle Layer:

• 5 Tbsp (67g) boiling water
• 2 Tbsp (10g) Dutch-process cocoa powder
• 7 oz (200g) bittersweet chocolate*, chopped fine
• 1-1/2 cups (340g) heavy cream, chilled
• 1 Tbsp sugar
• 1/8 tsp table salt

Top Layer:

• 1 Tbsp cold water
• 3/4 tsp unflavored gelatin powder
• 1 cup (6oz, 170g) white chocolate chips
• 1-1/2 cups (340g) heavy cream, chilled & divided

Optional: shaved semisweet chocolate shavings or cocoa powder for finishing

 

Instructions:

For the bottom later:

1. Adjust the oven rack to the middle position and preheat to 325F. Grease a 9” springform pan. Combine butter, chocolate, and espresso powder in a large heatproof bowl set over a saucepan with 1” of barely simmering water (make sure the water doesn’t touch the bottom of the bowl). Stir occasionally, until butter and chocolate are completely melted. Set aside and let cool 5 min. Whisk in egg yolks and vanilla.
2. Using a stand mixer fitted with a whisk**, whip the egg whites, cream of tartar, and salt together on medium-low until foamy (about 1 min). Add half the sugar (35.5g) and continue whisking 15-30 seconds. Add the remaining sugar (35.5g) and increase the speed to high, whipping until soft peaks form. Use a whisk to add a third of the egg whites to the chocolate mixture, stirring to slacken it. Then, using a spatula, gently but thoroughly fold the remaining egg whites into the mixture until no white streaks remain. Carefully pour the batter into the pan and smooth the top with a spatula.
3. Bake until cake has risen, is firm around the edges, and springs back when gently pressed in the center, 13-18 min. Let cake cool completely (about 1 hr) on wire rack. It will collapse as it cools – this is normal. Do not remove it from the pan.

For the middle layer:

1. Bloom the cocoa by combining it with the hot water; set aside. Melt the chopped chocolate in a large bowl set over a saucepan filled with 1” of barely simmering water (make sure the water doesn’t touch the bottom of the bowl). Remove from heat and cool, about 5 min.
2. Using a mixer with a whisk attachment, whip the cream, sugar, and salt on medium-low speed until foamy, about 1 min. Increase speed to high and whip until soft peaks form.
3. Whisk the bloomed cocoa into the melted chocolate until smooth. Using a whisk, fold a third of the whipped cream into the chocolate mixture to slacken it. Then, use a flexible spatula to gently fold in the remaining whipped cream until no white streaks remain. Gently spoon/pour mousse over the bottom cake layer and smooth the top with an offset spatula. Wipe the inside edge of the pan with a damp cloth or paper towel to clean up any drips. Refrigerate at least 30 min.

For the top layer:

1. In a small bowl, sprinkle gelatin over the water and allow to soften, about 5 min. Place chocolate chips in a medium heatproof bowl. In a small saucepan, bring 1/2 cup of the cream (113g) to a good simmer over medium heat. Remove from heat and add the gelatin mixture, stirring until the gelatin is dissolved. Pour the mixture over the chocolate chips and allow it to sit, covered, for 5 min. Whisk gently until smooth. Let cool completely (mixture will thicken slightly).
2. Using a mixer with a whisk attachment, whip the remaining 1 cup heavy cream (227g) on medium-low speed until foamy, about 1 min. *** Increase speed to high and whip until soft peaks form. Using a whisk, fold a third of the whipped cream into the white chocolate mixture to slacken it. Then, use a flexible spatula to gently fold in the remaining whipped cream until no white cream streaks remain. Spoon/pour the mousse over the chocolate mousse layer and smooth the top with an offset spatula. Refrigerate cake for at least 2-1/2 hours or up to 24 hours (let sit at room temp for 45 min before releasing from the pan and serving).

To finish/serve: Garnish the top of the cake with chocolate shavings or a dusting of cocoa powder, if using. Run a thin knife around the edge of the pan to loosen cake, the remove the sides of the pan. Run a clean knife around the outside of the cake to smooth. Slice and serve.

My notes:

• *The recipe headnote specifies Ghirardelli 60% bittersweet baking bars for the bottom two layers. This is what I used and definitely recommend too.
• **When whipping egg whites or making meringue, making sure the bowl and tools are free of any particles of fat is crucial. As such, I like to give the bowl and whisk attachment a quick wipe down with white vinegar before I start baking. If you don't feel confident in separating eggs, make sure to use a separate bowl so that you don't have to start over if a bit of yolk gets into the stand mixer.
• ***I chose to add a splash of vanilla and a sprinkle of table salt to the whipped cream at this stage.

If you decide to try it, please send me a message and let me know!
I hope you enjoy it as much as we did.

And don't forget to pick up a copy of this cookbook for more excellent recipes like this one!