Mocha Layer Cake with Chocolate-Rum Cream Filling

 This showstopping layer cake combines deep coffee-chocolate mocha cake with a silky, boozy chocolate-rum cream filling. The filling is a diplomat cream—a pastry cream lightened with whipped cream—so it's rich but not heavy. Finish with a simple chocolate ganache or mocha buttercream.

Yield: One 8-inch or 9-inch 3-layer cake
Prep time: 1 hour (plus chilling)
Bake time: 25–30 minutes

For the Mocha Cake Layers

Ingredient	Amount
All-purpose flour	2 ½ cups (315 g)
Granulated sugar	2 cups (400 g)
Unsweetened cocoa powder (natural or Dutch-process)	¾ cup (75 g)
Baking soda	2 tsp
Baking powder	1 tsp
Salt	1 tsp
Strong brewed coffee (cooled)	1 cup (240 ml)
Buttermilk (room temp)	1 cup (240 ml)
Vegetable oil	½ cup (120 ml)
Large eggs (room temp)	2
Vanilla extract	2 tsp

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For the Chocolate-Rum Cream Filling (Diplomat Cream)

Ingredient	Amount
Whole milk	1 ½ cups (360 ml)
Heavy cream (for pastry cream)	½ cup (120 ml)
Egg yolks	4 large
Granulated sugar	½ cup (100 g)
Cornstarch	3 tbsp (24 g)
Dark chocolate (60–70%), finely chopped	4 oz (115 g)
Unsalted butter (softened)	2 tbsp (28 g)
Dark rum (or spiced rum)	3 tbsp (45 ml)
Vanilla extract	1 tsp
Additional heavy cream (cold, for whipping)	1 cup (240 ml)
Powdered sugar (for whipped cream)	2 tbsp (15 g)

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For the Ganache Glaze (Optional but Recommended)

Ingredient	Amount
Semi-sweet or dark chocolate, chopped	6 oz (170 g)
Heavy cream	½ cup (120 ml)
Butter (optional, for shine)	1 tbsp (14 g)
Rum or vanilla extract	1 tsp

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Instructions

1. Make the Mocha Cake Layers

1. Preheat oven to 350°F (175°C). Grease and flour three 8-inch or 9-inch round cake pans, and line the bottoms with parchment.

2. In a large bowl, sift together flour, sugar, cocoa powder, baking soda, baking powder, and salt.

3. In another bowl, whisk together the cooled coffee, buttermilk, oil, eggs, and vanilla.

4. Pour the wet ingredients into the dry and mix on medium speed just until combined and smooth (about 1 minute). Do not overmix.

5. Divide batter evenly among the three pans. Bake for 25–30 minutes, until a toothpick comes out clean.

6. Cool in pans for 10 minutes, then turn out onto wire racks to cool completely. Level the tops with a serrated knife if needed.

2. Make the Chocolate-Rum Cream Filling

First, make the chocolate pastry cream base:

1. In a medium saucepan, heat the whole milk and the ½ cup heavy cream until just simmering (steaming, not boiling).

2. In a separate bowl, whisk egg yolks, sugar, and cornstarch until pale and thick.

3. Slowly drizzle about 1 cup of the hot milk mixture into the yolk mixture while whisking constantly (tempering). Then pour the tempered yolk mixture back into the saucepan.

4. Cook over medium heat, whisking constantly, until it thickens and just comes to a boil. Boil for 30 seconds, then remove from heat.

5. Add the chopped dark chocolate, butter, rum, and vanilla. Stir until smooth.

6. Pour the pastry cream into a shallow bowl, press plastic wrap directly onto the surface (to prevent a skin), and refrigerate until completely cold (at least 2 hours, or overnight).

Then, finish the diplomat cream:

1. Just before assembling the cake, whip the additional 1 cup cold heavy cream with the powdered sugar to stiff peaks.

2. Scrape the cold chocolate pastry cream into a large bowl and beat briefly with a mixer until smooth and pliable.

3. Fold the whipped cream into the pastry cream in two additions, using a rubber spatula. Fold gently until no streaks remain. The filling will be light, creamy, and mousse-like.

3. Make the Ganache Glaze (if using)

1. Place chopped chocolate in a heatproof bowl.

2. Heat heavy cream until just simmering, then pour over the chocolate. Let sit 2 minutes.

3. Whisk gently until smooth. Whisk in butter and rum for extra shine and flavor.

4. Let cool until slightly thickened but still pourable (about 15–20 minutes).

4. Assemble the Cake

1. Place the first mocha cake layer on a serving plate or cake board. Pipe or spread a ½-inch-thick layer of the chocolate-rum cream filling right to the edge.

2. Top with the second cake layer, press lightly, and repeat with more filling.

3. Add the third cake layer (bottom side up for a flat top).

4. Crumb coat (optional): Spread a thin layer of ganache or buttercream over the entire cake to seal in crumbs. Chill 20 minutes.

5. Pour the remaining ganache over the top, letting it drip down the sides. Smooth with an offset spatula. Or, if using mocha buttercream, frost as desired.

6. Chill the assembled cake for at least 1 hour before slicing. (The filling needs to set up firmly.)

Serving & Storage

• Serving: Let the cake sit at room temperature for 20 minutes before slicing. Use a warm, dry knife and wipe clean between cuts.

• Storage: Refrigerate leftovers in an airtight container for up to 4 days. The filling contains eggs and dairy, so do not leave at room temperature for more than 2 hours.

• Make-ahead: The cake layers can be wrapped and frozen for up to 1 month. The pastry cream base can be made 2 days ahead. Complete the filling and assemble the day of serving for the best texture.

Notes & Substitutions

• No rum? Substitute with 1 teaspoon rum extract plus 2 tablespoons milk or coffee. Or use bourbon, brandy, or omit entirely for a non-alcoholic version.

• Coffee: Instant espresso powder works beautifully—dissolve 2 tablespoons in 1 cup of hot water instead of brewed coffee.

• Lighter filling: Skip the pastry cream and use chocolate-rum whipped cream: whip 2 cups cold heavy cream with ¼ cup powdered sugar, ¼ cup cocoa powder, and 2 tablespoons rum. But the diplomat cream is worth the extra step.

• Frosting alternative: For a fully frosted cake, pair with mocha Swiss meringue buttercream or a simple coffee-vanilla buttercream.

Enjoy your indulgent, coffee-kissed, rum-spiked layer cake!