Aidan's birthday was yesterday, and every year, I try to make him a different type of cake. This year, I made him a dark chocolate mousse cake. This recipe is a bit "involved," but totally worth your time. Check out the recipe below:

Chocolate Cake

  • 1 and 3/4 C all-purpose flour
  • 3/4 C unsweetened natural cocoa powder
  • 1 and 3/4 C granulated sugar
  • 2 tsp baking soda
  • 1 tsp baking powder
  • 1 tsp salt
  • 1/2  C vegetable oil
  • 2 Large eggs, at room temperature
  • 3/4 C fat sour cream, at room temperature
  • 1/2 C buttermilk, at room temperature*
  • 2 tsp pure vanilla extract
  • 1/2 C hot water or coffee*
  • optional: 1 C mini or regular semi-sweet chocolate chips
  • optional garnish: fresh berries and/or chocolate shavings

Chocolate Mousse

  • 1/2 C hot water
  • 1/4 C unsweetened cocoa powder
  • Two 4-ounce quality semi-sweet chocolate bars, finely chopped
  • 2 C heavy cream or heavy whipping cream
  • 2 Tbsp confectioners’ sugar
  • 1/2 tsp pure vanilla extract

Chocolate Ganache

  • Two 4 oz quality semi-sweet chocolate bars, finely chopped
  • 1 C heavy cream or heavy whipping cream

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Instructions

  1. Preheat oven to 350°F. Grease three 8-inch, line with parchment paper, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans.
  2. Make the cake: Whisk the flour, cocoa powder, sugar, baking soda, baking powder, and salt together in a large bowl. Set aside.
  3. Using a handheld or stand mixer fitted with a whisk attachment (or you can use a whisk) mix the oil, eggs, and sour cream together on medium-high speed until combined. Add the buttermilk and vanilla and mix until combined.
  4. Pour the wet ingredients into the dry ingredients, add the hot water/coffee, and whisk or beat on low speed until the batter is completely combined. Fold in the chocolate chips, if using.
  5. Divide batter evenly among 3 pans. Bake for 19-23 minutes. Baking times vary, so keep an eye on yours. The cakes are done when a toothpick inserted in the center comes out clean.
  6. Remove the cakes from the oven and set on a wire rack. Allow to cool completely in the pan.
  7. As the cakes cool, prepare the chocolate mousse so it can chill and be ready at the same time as the cake layers. You can also prepare the mousse 1-2 days ahead of time. Whisk the hot water and cocoa powder together. Set aside. Melt the chopped chocolate bars in the microwave. Use a heatproof bowl and stir every 20 seconds, until the chocolate is smooth. Pour hot water/cocoa mixture into melted chocolate and stir until thick and smooth. Set aside.
  8. Using a hand mixer or a stand mixer fitted with a whisk attachment, whip the heavy cream, confectioners’ sugar, and vanilla extract together on medium-high speed until medium peaks form, about 3-4 minutes. Medium peaks are between soft/loose peaks and stiff peaks.
  9. Pour in the chocolate mixture and using a spoon or rubber spatula, gently fold together. Avoid over-mixing which can deflate/thin out the mousse. Cover mousse and chill in the refrigerator for at least 2 hours and up to 2 days. Makes about 4.5 cups chocolate mousse.
  10. Assemble layers before preparing chocolate ganache: First, level the cakes if needed: using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). 
  11. Place 1 cake layer on your cake stand or serving plate. Using a large icing spatula, evenly cover the top with about 2 cups chocolate mousse. Top with 2nd layer and evenly cover the top with 2 cups chocolate mousse. Save a large spoonful of mousse for a thin crumb coat.
  12. Spread any remaining mousse around the sides as a crumb coat. Run a bench scraper around the cake to smooth out crumb coat.
  13. Refrigerate cake for at least 1 hour and up to 4 hours. Prepare the ganache as you wait.
  14. Chocolate Ganache: Place chopped chocolate in a medium heat-proof bowl. Heat the cream in a small saucepan over medium heat until it begins to gently simmer. (Do not let it come to a rapid boil– that’s too hot!) Pour over chocolate, then let it sit for 2-3 minutes to gently soften the chocolate. With a metal spoon or small rubber spatula, very slowly stir until chocolate has melted and mixture is smooth. The finer you chopped the chocolate, the quicker it will melt with the cream. If it’s not melting, do not microwave it. Once ganache mixture is smooth, let it cool for 20 minutes at room temperature before spreading on chilled cake.
  15. Pour/spoon ganache on chilled cake. Smooth the top with an icing spatula and the sides with a bench scraper. Top with optional garnish such as fresh berries and/or chocolate shavings. Serve cake immediately or chill, uncovered, for up to 4-6 hours before serving. Cake can be served at room temperature or chilled.
  16. Cover leftover cake tightly and store in the refrigerator for 5 days.

Notes

  1. Make Ahead & Freezing Instructions: Prepare cake through step 4. Wrap the individual baked and cooled cake layers tightly and refrigerate for up to 2 days or freeze up to 3 months. Bring to room temperature, make mousse filling, then continue with step 6. You can prepare the chocolate mousse in advance. See step 5. You can also prepare the chocolate ganache ahead of time too. Refrigerate prepared ganache for up to 2-3 days. Bring to room temperature before spreading onto cake. Frosted cake freezes well, up to 3 months. Thaw overnight in the refrigerator, then bring to room temperature or serve cold.
  2. Why Room Temperature? All refrigerated items should be at room temperature so the batter mixes together easily and evenly. Instead of sour cream, you can use plain yogurt. The cake won’t taste as rich, but it’s a fine substitute. If needed, you can use whole milk mixed with 1/2 teaspoon of white vinegar or fresh lemon juice instead of buttermilk.
  3. Chocolate in Mousse & Ganache: Mousse & ganache will only set if the correct chocolate is used. You can use high quality chocolate chips if needed (I prefer Ghirardelli semi-sweet chocolate chips), but I recommend using pure chocolate baking bars. You can find them right next to the chocolate chips in the baking aisle.