Most delicious cake I've ever tasted! But make sure you have enough room in your refrigerator or even freezer for the two cakes and the mousse. Also make sure you can take you're time to make it. It may take 2 hrs but it's definitely worth it!
Preheat your oven to 350°F. Place a rack in the lower third of the oven. You will need two 8 or 9inch round pans, spray with Bakers joy cooking spray, this is a sticky cake!
Take a medium size sauce pan and add your sugar, cocoa, baking soda and water to it. Put it over medium - high heat and bring to a boil. Stir occasionally to prevent sticking. After bringing to a boil set aside for 10mins. WARNING : ONCE IT HAS COME TO A BOIL IT WILL RISE ALMOST OUT OF THE POT! SO INSTANTLY REMOVE FROM HEAT!
While waiting on the cocoa mixture, get all the other cake ingredients together. Put your oil and some vanilla (eye ball it) in your electric mixer(this can all be done by hand with a wooden spoon if need be) mix them on low.
After the 10mins are up, stir in expresso powder into your cocoa mixture.(this is to help keep a dark rich color to the cake and doesn't give it a coffee flavor) Then pour cocoa mixture into mixer, mix using lowest speed(stir). Add your flour, egg yolks, eggs and mix well. Add the vanilla and butter milk. Mix well, batter will be runny.
Pour batter into your two pans and place in oven for 25-30 mins or until the tops of the cakes no longer jiggle.
While waiting for your cakes to cook go ahead and prepare your mousse and refrigerate! This is very important for when you put the cake together, you want it firm so the cakes don't slide off one another.
After the cakes are done allow them to cool for about 10-15mins then place them in the refrigerator (this also helps when putting the cake together, the cooler everything is the less likely the cake will be to slide apart! Yes this comes from experience)
While waiting for your cakes to cool go ahead and make your ganache. Put your heavy whipping cream and butter into a sauce pan and bring to a boil. Put your chocolate chips in a bowl. After your cream and butter have come to a boil pour over your chocolate chips, make sure they're submerged and let sit for 5mins. Do not stir. While you wait take your cakes and mousse out(make sure they are cool) and layer the cake with the mousse in the middle saving some for decorative topping. Once the gaunche has sit for 5mins, stir it up and pour it over the top of the cake. Decorate with the left over mousse then Refrigerate until ready to serve.