Making the Chocolate Mousse Cake is a straightforward process if you follow these easy steps:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
2. Mix Dry Ingredients: In a bowl, sift together the flour, cocoa powder, baking powder, and salt. Set aside.
3. Beat Egg Yolks: In a separate bowl, whisk the egg yolks and sugar until pale and creamy.
4. Add Butter and Vanilla: Stir in the melted butter and vanilla extract into the egg mixture.
5. Combine Mixtures: Gradually add the dry ingredients to the egg mixture, stirring until just combined.
6. Whip Egg Whites: In another bowl, beat the egg whites until stiff peaks form.
7. Fold in Egg Whites: Gently fold the whipped egg whites into the cake batter, being careful not to deflate them.
8. Bake: Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a toothpick inserted comes out clean.
9. Cool the Cake: Once baked, remove from the oven and let it cool in the pan for 10 minutes. Then, transfer to a wire rack to cool completely.
10. Prepare the Mousse: In a saucepan, heat the heavy cream until just simmering. Remove from heat and add the chopped chocolate, stirring until melted and smooth.
11. Whip the Cream: Allow the chocolate mixture to cool slightly, then whip the remaining heavy cream until soft peaks form. Fold the whipped cream into the chocolate mixture.
12. Assemble: Once the cake is completely cool, slice it in half horizontally. Spread a layer of chocolate mousse between the two layers and then cover the top and sides of the cake with the remaining mousse.
13. Chill: Place the assembled cake in the refrigerator for at least 1 hour to set.

These steps will guide you to create a truly incredible Chocolate Mousse Cake that will impress your family and friends!