Step 1: Prepare your bundt pan. Generously grease and flour a 10-12 cup bundt pan, ensuring every crevice is coated to guarantee a clean release.
Step 2: Make the chocolate cake batter (dry ingredients). In a large bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking soda, baking powder, and salt. Set aside.
Step 3: Make the chocolate cake batter (wet ingredients). In another bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
Step 4: Combine wet and dry for cake batter. Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix, as this can lead to a dry cake.
Step 5: Layer cake batter. Pour about two-thirds of the chocolate cake batter into your prepared bundt pan. Smooth the top gently with a spatula.
Step 6: Prepare the mousse base. In a medium saucepan, combine milk, granulated sugar, and cocoa powder. Whisk until smooth and heat over medium-low heat until it just begins to simmer, stirring frequently. Remove from heat.
Step 7: Temper egg yolks. In a separate bowl, whisk egg yolks until pale. Gradually temper the hot chocolate mixture into the egg yolks, whisking constantly to prevent the eggs from scrambling.
Step 8: Thicken mousse base. Return the entire mixture to the saucepan and cook over low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (about 3-5 minutes). Do not boil.
Step 9: Melt chocolate and cool. Remove from heat and stir in the chopped semi-sweet chocolate until melted and smooth. Let cool slightly.
Step 10: Fold in whipped cream. In a separate cold bowl, whip the very cold heavy cream to medium peaks. Gently fold the cooled chocolate mixture into the whipped cream until no streaks remain. This creates the creamy mousse for your chocolate swirl mousse bundt cake recipe.
Step 11: Assemble and swirl. Carefully spoon dollops of the prepared chocolate mousse over the cake batter already in the bundt pan. Then, spoon the remaining chocolate cake batter over the mousse. Using a knife or skewer, gently swirl through the layers 2-3 times to create the marble effect, being careful not to overmix or disturb the pan's bottom. You want distinct layers and swirls, not a fully mixed batter.
Step 12: Bake the bundt cake. Bake in a preheated oven at 350°F (175°C) for 50-60 minutes, or until a wooden skewer inserted into the cake comes out with moist crumbs, not wet batter.
Step 13: Initial cooling. Let the cake cool in the pan on a wire rack for 15-20 minutes. This cooling period is crucial for the cake to set and ensures it releases cleanly.
Step 14: Final cooling and glazing. Invert the bundt cake onto a wire rack to cool completely. Once fully cooled, prepare the optional chocolate ganache glaze: heat 1/2 cup heavy cream until simmering, then pour over 4 oz chopped semi-sweet chocolate. Let sit for 5 minutes, then whisk until smooth. Drizzle generously over the cooled chocolate swirl mousse bundt cake.