Prepare the Chocolate Cake

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans, or line the bottoms with parchment paper.
2. Mix the Dry Ingredients: In a large bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
3. Add Wet Ingredients: Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Use a mixer on medium speed until everything is smooth and well combined.
4. Add the Boiling Water: Carefully pour in the boiling water, mixing until just combined. The batter will be very thin, but that’s what makes the cake so moist.
5. Bake the Cakes: Divide the batter evenly between the three prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
6. Cool the Cakes: Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Prepare the Chocolate Mousse Filling

1. Melt the Chocolate: Place the chopped semi-sweet chocolate in a heatproof bowl. In a small saucepan, heat 1 cup of heavy cream over medium heat until it just begins to simmer. Pour the cream over the chopped chocolate and let it sit for 1 minute.
2. Mix: Stir until the chocolate is completely melted and smooth. Let the mixture cool to room temperature.
3. Whip the Cream: In a separate bowl, whip 1 cup of heavy cream with 2 tbsp of sugar until stiff peaks form.
4. Combine: Gently fold the whipped cream into the cooled chocolate mixture until smooth and fluffy.

Prepare the Chocolate Ganache

1. Heat the Cream: In a small saucepan, heat 1 cup of heavy cream over medium heat until it begins to simmer.
2. Make the Ganache: Pour the hot cream over the chopped dark chocolate and let it sit for 1 minute. Stir until smooth and glossy.
3. Add Butter: Stir in the butter for extra shine and richness. Let it cool for a few minutes before pouring it over the cake.

Assemble the Cake

1. Layer the Cake: Place the first cake layer on a cake stand or platter. Spread half of the chocolate mousse evenly over the top.
2. Add the Second Layer: Place the second cake layer on top and spread the remaining mousse on top.
3. Top with the Third Layer: Place the third cake layer on top, pressing gently to ensure the layers are even.
4. Pour the Ganache: Pour the ganache over the top of the cake, allowing it to drip down the sides for a beautiful, glossy finish.

Chill the Cake: Refrigerate the cake for at least 1 hour to set the mousse and ganache before serving.