Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans.

In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix well.

Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes.

Stir in the boiling water until the batter is smooth (the batter will be thin). Pour evenly into the prepared cake pans.

Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then remove from pans and cool completely on wire racks.

For the mousse, melt the chopped chocolate in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and let cool slightly.

In a separate bowl, beat the egg yolks and sugar together until thick and pale. Stir in the melted chocolate and vanilla extract.

In another bowl, whip the heavy cream until soft peaks form. Gently fold the whipped cream into the chocolate mixture.

In a clean bowl, beat the egg whites until stiff peaks form. Carefully fold the egg whites into the chocolate mixture until no white streaks remain.

Once the cakes are completely cool, place one layer on a serving plate. Spread half of the mousse over the first layer, then top with the second cake layer.

Spread the remaining mousse on top and around the sides of the cake. Refrigerate for at least 4 hours or overnight for best results.