Step 1: Make the Chocolate Cake

1. Preheat oven to 350°F (175°C). Grease and line a 9-inch round cake pan with parchment paper.
2. In a large bowl, whisk together flour, baking powder, baking soda, salt, cocoa powder, brown sugar, and granulated sugar.
3. Add oil, buttermilk, sour cream, eggs, vinegar, and vanilla extract. Mix until smooth.
4. Stir in hot water until fully combined. The batter will be thin.
5. Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
6. Let the cake cool completely before adding the mousse layer.

Step 2: Prepare the Chocolate Mousse

1. In a bowl, whisk together instant pudding mix, milk, and cornstarch until thickened.
2. In a separate bowl, beat heavy cream until soft peaks form.
3. Gently fold the whipped cream into the pudding mixture until smooth.
4. Spread the mousse evenly over the cooled chocolate cake. Refrigerate for at least 1 hour to set.

Step 3: Make the Chocolate Whipped Cream

1. In a chilled mixing bowl, beat heavy cream, powdered sugar, vanilla, and cocoa powder until stiff peaks form.
2. Spread or pipe the whipped cream over the mousse layer.

Step 4: Chill & Serve

1. Refrigerate the cake for at least 4 hours (or overnight for best results).
2. Slice and serve! Enjoy this rich, creamy chocolate mousse cake.