Prepare the Cake Pan and Oven: Start by preheating the oven to 350°F (175°C). Line the bottom of an 8-inch round cake pan with parchment paper and lightly grease the sides. This preparation helps the cake release smoothly later without sticking.
Mix the Dry Ingredients: In a medium bowl, whisk together flour, cocoa powder, sugar, baking powder, and salt. Mixing these ingredients first helps distribute the cocoa evenly so the cake develops a consistent chocolate flavor throughout.
Combine the Wet Ingredients: In another bowl, whisk the eggs, oil, milk, and vanilla extract until smooth. Slowly stir this mixture into the dry ingredients until a thick but smooth chocolate batter forms.
Bake the Cake Base: Pour the batter into the prepared cake pan and spread it evenly. Bake for about 18–22 minutes until a toothpick inserted in the center comes out with a few moist crumbs. Allow the cake to cool completely before adding the mousse layer.
Melt the Dark Chocolate: Place chopped dark chocolate in a heatproof bowl and melt it gently over a bowl of warm water or in short microwave intervals. Stir slowly until smooth and glossy, then allow it to cool slightly before mixing with the whipped cream.
Whip the Cream: In a large mixing bowl, beat the heavy cream with powdered sugar and vanilla until soft peaks form. The texture should feel airy and light while still holding its shape.
Fold Chocolate into Cream: Gently fold the melted chocolate into the whipped cream using a spatula. Mix slowly with light movements so the mousse keeps its airy texture while becoming evenly chocolatey.
Assemble and Chill the Cake: Spread the mousse evenly over the cooled cake layer while it remains in the pan. Smooth the surface with a spatula and refrigerate for at least 3 hours so the mousse can firm up into a silky layer.
