Mocha Tiramisu Layer Cake
Espresso, rum, chocolate — the three things that are most enchanting in tiramisu and mocha and the three flavors that my dad loves the most in desserts. I made this layered cake for his birthday earlier this year, and the cake was the brainchild of multiple different recipes and textures we both love. The two layers of cake are chocolate sponge cakes, soaked in rum syrup. A thick layer of coffee mousse runs between the cakes and brings a kick of moisture and warm coffee. Finally, the top has a chiseled top of ganache. While experimenting, I learned how ganache is the food equivalent of a chameleon: it adopts a new state at each temperature, just like a chameleon adapts to its surrounding. For a chiseled, marble-statue texture, I chilled the ganache for 2 hours, frosted it in a smooth disk on top of the cake, flash-froze it for 15 minutes, then used a hot offset spatula to swipe the chocolate in a spiral.
A light sponge cake may not be architecturally sound for heftier cakes used in exquisite sculptures, but it can hold up a layer of mousse. Using mousse as a center band is also not architecturally sound, unless the cake above it is equally airy. Despite its luxurious look, the cake is like eating air, with flavors and textures fleeting your mouth… until you realize you’re already on your third slice. This is definitely one of the longer recipes I’ve created, but, boy, is it a wonderful cake for a special event.
The ganache recipe is graciously adapted from Dominique Ansel’s recipe for chocolate truffles.
Ingredients:
chocolate sponge:
6 large eggs (50-60g each)
200g granulated sugar
1 tbsp vanilla extract
100g all purpose flour
35g dutched cocoa powder
1 tsp baking powder
1/2 tsp fine sea salt
rum soaking liquid:
2 tbsp (8g) light or dark rum
30g cold water
10g granulated sugar
coffee mousse:
2 egg yolks (30-35g each)
80g sugar
12g cornstarch
160ml warm milk
1 tsp (6g) vanilla extract
2 1/2 tbsp (10g) instant coffee
1 1/2 gelatin sheets
25g cold water
240ml chilled heavy cream, 35% fat
chocolate ganache:
220g heavy cream
45g whole milk
220g dark chocolate (I used Valrhona’s Manjari 64% Feves)
Steps:
Make the chocolate sponge. Preheat oven to 350º F. Line the bottom of 2 6-inch cake pans with parchment.
In a mixing bowl, beat eggs, sugar, and vanilla on high speed for 9 minutes until the eggs are thick, pale, and fluffy.
In another medium sized bowl, sift together the flour, cocoa powder, baking powder, and salt. Then sift the dry by third increments into the eggs. Make sure to gently fold the flour and mix from the bottom of the mixing bowl to avoid deflating the eggs.
From a height, pour cake batter into the prepared cake pans, dividing the batter evenly. Bake the cake for around 20 minutes. Immediately remove the cake from the oven and run a thin knife along the edge of the pan to release the cake. Let cool completely.
Make the rum soaking liquid. Stir rum, cold water, and sugar in mixing cup until the sugar has completely dissolved.
Then, with a cake leveler, split the cake in half, reserving the bottom layers as your 2 cake layers. With a brush, gently dab the rum soaking liquid over both layers. Cover both layers with plastic wrap and set aside.
Taking a tall 6 inch cake pan with a removable base. Line the cake pan with plastic wrap so that the ears hang at least 3 inches out of pan. Place one layer of cake down at the bottom of the pan.
Make the coffee mousse. Dissolve gelatin sheets in cold water and let it swell 5 to 10 minutes until the sheets are plump.
In a small saucepan with no heat, whip egg yolks with sugar until frothy/creamy and pale yellow. Whip in cornstarch. Whisk in warm milk slowly until there are no lumps.
Place saucepan over low heat and stir constantly until mixture boils and thickens. Remove from heat and vigorously stir and quash any lumps that may have formed in the custard. Stir in vanilla extract and instant coffee. Let cool.
Place gelatin over low heat until it just dissolves and pour into the coffee mixture. Stir completely.
Whip the chilled heavy cream in a clean bowl until stiff peaks form. Gently fold into the coffee mixture in thirds, being careful of deflating the heavy cream.
Spread coffee mousse over the chocolate sponge in the pan and smooth out the top with an offset spatula. Gently bang the cake pan on the counter to remove any air bubbles in the coffee mouse.
Place the second layer of chocolate sponge on top of the mouse and fold the dangling ears of the plastic wrap over the top layer of the cake. With a new piece of plastic wrap, cover the cake pan.
Refrigerate for at least twelve hours.
Make the ganache. Mix cream and milk in a small saucepan and bring to a low boil over medium heat, stirring constantly. Remove from heat.
Place the chocolate in a large bowl and gradually pour the cream mixture over the chocolate and stir until the chocolate has melted and is completely smooth. Only mix to melt, not build air bubbles. Tap the bowl gently against the counter to rid any potential air bubbles. Chill in fridge for 2-3 hours until ganache reaches a malleable, soft, clay-like consistency.
Remove the cake from the fridge and lift from plastic wrap. Place the cake on a cake stand. With an offset spatula, dollop ganache on top of the cake and frost to desired height and texture.
Let set in fridge for another 10-15 minutes, and enjoy!