Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper to prevent sticking.

In a mixing bowl, combine the sifted flour, granulated sugar, sifted cocoa powder, and salt. Whisk together until well combined.

In another bowl, whisk together the melted butter, eggs, and vanilla extract until smooth.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix to keep the cake tender.

Pour the batter into the prepared cake pan and smooth the top with a spatula.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Once baked, remove from the oven and let the cake cool in the pan for 10 minutes. Then, transfer to a cooling rack to cool completely.

In a double boiler or microwave-safe bowl, melt the chopped dark chocolate until smooth. If using a microwave, heat in 30-second intervals, stirring in between to prevent burning.

In a mixing bowl, whisk the egg yolks with the granulated sugar until pale and creamy. Gradually add the melted chocolate to the egg yolk mixture, stirring until well combined.

In a separate bowl, whip the chilled heavy cream until soft peaks form. This will add lightness to the mousse.

Gently fold the whipped cream into the chocolate mixture using a rubber spatula. Be careful not to deflate the whipped cream, as this keeps the mousse airy.

In another bowl, whisk the egg whites until stiff peaks form. This will provide additional structure to the mousse.

Gently fold the egg whites into the chocolate mixture in three additions, ensuring everything is well combined but still light and fluffy.

Once the cake has cooled completely, carefully slice it in half horizontally to create two layers.

Place one layer of the cake on a serving plate. Spread half of the chocolate mousse over the first layer.

Top with the second layer of cake and spread the remaining mousse on top, smoothing it out with a spatula.

Refrigerate the assembled cake for at least 2 hours to allow the mousse to set properly.

In a mixing bowl, whip the heavy cream with powdered sugar until stiff peaks form. This will be used for decoration.

Pipe or spread the whipped cream on top of the mousse layer. Garnish with chocolate shavings for an elegant finish.

Slice the cake into wedges and serve chilled. Enjoy the rich, creamy texture of the mousse with the lightness of the cake base.