1. Preheat your oven to 350°F. Grease two 9-inch round cake pans and line the bottoms with parchment paper.
2. In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
3. Add the eggs, milk, oil, and vanilla extract to the dry ingredients and mix until well combined.
4. Stir in the boiling water until the batter is smooth. It will be quite thin, but this is okay.
5. Divide the batter evenly between the prepared pans and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
6. While the cakes are cooling, whip the heavy cream and powdered sugar in a large bowl until soft peaks form.
7. Fold in the melted chocolate gently, ensuring you maintain the lightness of the mousse.
8. Once the cakes are completely cool, spread a thick layer of mousse between the layers and on top of the cake.
9. Refrigerate for at least 2 hours or until the mousse is set and the cake is chilled through.

As someone who has seen both triumphs and mishaps in the kitchen, I can tell you that patience is key here. Letting the mousse set properly is what transforms this cake into a showstopper.