Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 10-12 cup bundt pan thoroughly. Don't underestimate this step; a well-greased and floured pan is your best friend for a clean release.
Step 2: In a large bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking soda, and salt. Set aside.
Step 3: In a separate bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy. Add the large eggs one at a time, beating well after each addition. Stir in 1 teaspoon of vanilla extract.
Step 4: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry. Mix until just combined. Do not overmix! Overmixing develops gluten, which can lead to a tough cake.
Step 5: Pour the batter into the prepared bundt pan and spread evenly. Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 6: Let the cake cool in the pan on a wire rack for 10-15 minutes before inverting it onto the rack to cool completely. Cooling completely is crucial before adding the mousse.
Step 7: Finely chop the dark chocolate. In a heatproof bowl, combine the chopped chocolate with 1/2 cup of heavy cream. Heat gently over a double boiler or in the microwave in 20-second intervals, stirring until smooth and completely melted. Let it cool to room temperature. This is your chocolate ganache base for the mousse.
Step 8: In a very cold bowl (chilled for at least 15 minutes), whip the remaining 1.5 cups of cold heavy cream with powdered sugar and 1 teaspoon of vanilla extract until medium-stiff peaks form. Be careful not to over-whip, or you'll end up with butter!
Step 9: Gently fold about one-third of the whipped cream into the cooled chocolate mixture to lighten it. Then, fold in the remaining whipped cream until no streaks remain. Be gentle to maintain the airy texture.
Step 10: Once the bundt cake is completely cool, use a small knife or spoon to carefully scoop out a channel from the bottom of the cake, leaving a 1-inch border on the sides. Be gentle to avoid breaking the cake. This creates the perfect space for your rich mousse.
Step 11: Spoon or pipe the chocolate mousse into the channel you created. Smooth the top if needed.
Step 12: Carefully place the cake (mousse side up) onto a serving platter. Chill the Chocolate Mousse Bundt Cake Recipe in the refrigerator for at least 2-3 hours, or preferably overnight, to allow the mousse to set properly and the flavors to meld.