In a medium bowl, combine the eggs, sugar, cocoa powder, cornstarch and salt. Whisk them together until the mixture is smooth; set aside.

In a medium saucepan, over low-medium heat, bring the milk to a simmer and then slowly stream the milk mixture over the egg mixture, while whisking continuously.

Return the whole mixture to the pot and over low-medium heat, whisk continuously for about five minutes, until the pastry cream starts to hold a streak from the whisk.

Whisk more aggressively for about 30-60 more seconds and then pour it into a bowl (large shallow bowl will cool quicker). 

Add the butter and vanilla and continue whisking for another minute, until the butter is fully combined and then cover with a lid or plastic wrap to cool completely. 

In a large bowl or the bowl of a stand mixer, use a whisk attachment to beat together the egg whites and granulated sugar.

Beat for a few minutes at full speed, until the meringue is fluffy and holds stiff peaks.

Add the egg yolks, melted butter and vanilla extract and beat at medium speed, just until they’re combined.

Sift 1/2 of the flour into the egg mixture and use a rubber spatula to gently fold it into the batter.

Sift the other half of the flour and salt over the batter and fold just until it’s combined. 

Pour the batter into the prepared pans, about 10 ounces each (or  6.5 ounces if making in three pans) and bake for about 20 minutes (maybe about 5 minutes less if baking in three pans). The tops should be golden brown and spring back when you gently press on the center. 

Remove the cakes from the oven and run a knife around the edge of the cake to release it from the pan. Allow it to cool for 15 minutes in the pan and then remove the cakes and place them onto a cooling rack or a tea towel. 

Allow the cake to cool completely and then use a large serrated knife to slice each layer into threes (or if you baked three shorter cakes, slice them each in half). 

Beat the softened butter for 5 minute at full speed, until it’s pale and fluffy.

Add in the pastry cream in 4-5 increments, mixing fully between each addition. 

Use a rubber spatula to smooth out the cream when it’s finished.

Place the first layer of cake onto a flat surface and spread a thin layer of buttercream on top. Layer the next cake and repeat until it's all stacked.

Pour on top of the chilled cake by moving your hand around while pouring it. Then tilt the cake so the chocolate spreads all over the top and drips off the edge.

You can also use an offset spatula to spread the chocolate but you'll lose the sheen and end up with a matte finish on the chocolate instead if you touch it.