Bake the chocolate cake layers

• Preheat the oven and grease and line three 8-inch cake pans.
• Whisk the flour and cocoa powder together.
• Beat the eggs with the sugar until pale and fluffy, then fold in the dry ingredients.
• Divide the batter between the pans and bake until a skewer comes out clean.
• Let the layers cool completely on a cooling rack.

Make the 3X chocolate mousse

• Melt the dark chocolate and let it cool slightly.
• Whip the cold heavy cream to soft peaks.
• Fold the melted chocolate into the whipped cream in three additions until smooth and airy.
• Chill the mousse until firm enough to spread.

Make the buttercream frosting

• Beat the softened butter until pale and creamy.
• Gradually add the powdered sugar and beat until light and fluffy.
• Add the vanilla extract and mix well.
• Tint the frosting with black food coloring until evenly dark.

Assemble the cake

• Place the first cake layer on a cake stand and spread a layer of chocolate mousse.
• Repeat with the remaining layers and mousse.
• Cover the whole cake with the black buttercream frosting and smooth it with a spatula.
• Chill the cake before serving.