Preheat the oven to 350F and line two 9” baking pans.
Whisk eggs and sugar at high speed for 5-7 minutes until light and thick. Sift in cocoa powder and flour, then carefully fold until just combined. Divide the batter equally between the two pans and bake for 12 minutes.
For the dark chocolate mousse, start by making ganache: heat the cream until it starts to bubble, then pour it over the chocolate. Cover and let it sit for 5 minutes, then mix until shiny. Whip heavy cream and fold it into the ganache once it’s slightly cooled (about 15 minutes).
Repeat the process for the milk chocolate mousse.
Allow everything to cool for 30 minutes. Meanwhile, make the ganache following the same method described above and let it cool.
Make the syrup by boiling water and sugar until syrupy. Slice the two cakes into two halves each and start assembling. Place one layer, brush with syrup, cover with half of the dark chocolate mousse. Repeat, alternating between dark and milk chocolate mousse. Smooth the edges and chill the cake for 15 minutes.
Cover with ganache and carefully smooth the edges. Cool as needed.
Make the syrup by making ganache and mixing until combined. Add corn syrup and mix.
To glaze, the cake should be cool, and the glaze should be at room temperature and runny (warm it slightly if not runny). Pour over the cake and smooth. Let it cool for 15 minutes.
Enjoy!